openssl pkcs8 -topk8 -in server-key.pem -out server-keyPkcs8Enc.pem

Step 1: Load in a pem formatted private key - OK supported in wolfSSL
Step 2: Convert PEM key to DER key - OK supported in wolfSSL
Step 3: Create DER formatted PKCS8 object from key - OK supported in wolfSSL
Step 4: Encrypt the PKCS8 object - NOT SUPPORTED
Step 5: Convert DER formatted PKCS8 object to PEM - NOT SUPPORTED

openssl -pkcs8 -nocrypt -topk8 -inform pem -in server-key.pem -outform der -out server-keyPkcs8.der

#include <stdio.h>

#include <wolfssl/options.h>
#include <wolfssl/ssl.h>
#include <wolfssl/wolfcrypt/types.h>
#include <wolfssl/wolfcrypt/rsa.h>
#include <wolfssl/wolfcrypt/asn.h>
#include <wolfssl/wolfcrypt/asn_public.h>

byte key[] = "\n\
-----BEGIN RSA PRIVATE KEY-----\n\
MIIEpQIBAAKCAQEAwJUI4VdB8nFtt9JFQScBZcZFrvK8JDC4lc4vTtb2HIi8fJ/7\n\
qGd//lycUXX3isoH5zUvj+G9e8AvfKtkqBf8yl17uuAh5XIuby6G2JVz2qwbU7lf\n\
P9cZDSVP4WNjUYsLZD+tQ7ilHFw0s64AoGPF9n8LWWh4c6aMGKkCba/DGQEuuBDj\n\
xsxAtGmjRjNph27Euxem8+jdrXO8ey8htf1mUQy9VLPhbV8cvCNz0QkDiRTSELlk\n\
wyrQoZZKvOHUGlvHoMDBY3gPRDcwMpaAMiOVoXe6E9KXc+JdJclqDcM5YKS0sGlC\n\
Qgnp2Ai8MyCzWCKnquvE4eZhg8XSlt/Z0E+t1wIDAQABAoIBAQCa0DQPUmIFUAHv\n\
n+1kbsLE2hryhNeSEEiSxOlq64t1bMZ5OPLJckqGZFSVd8vDmp231B2kAMieTuTd\n\
x7pnFsF0vKnWlI8rMBr77d8hBSPZSjm9mGtlmrjcxH3upkMVLj2+HSJgKnMw1T7Y\n\
oqyGQy7E9WReP4l1DxHYUSVOn9iqo85gs+KK2X4b8GTKmlsFC1uqy+XjP24yIgXz\n\
0PrvdFKB4l90073/MYNFdfpjepcu1rYZxpIm5CgGUFAOeC6peA0Ul7QS2DFAq6EB\n\
QcIw+AdfFuRhd9Jg8p+N6PS662PeKpeB70xs5lU0USsoNPRTHMRYCj+7r7X3SoVD\n\
LTzxWFiBAoGBAPIsVHY5I2PJEDK3k62vvhl1loFk5rW4iUJB0W3QHBv4G6xpyzY8\n\
ZH3c9Bm4w2CxV0hfUk9ZOlV/MsAZQ1A/rs5vF/MOn0DKTq0VO8l56cBZOHNwnAp8\n\
yTpIMqfYSXUKhcLC/RVz2pkJKmmanwpxv7AEpox6Wm9IWlQ7xrFTF9/nAoGBAMuT\n\
3ncVXbdcXHzYkKmYLdZpDmOzo9ymzItqpKISjI57SCyySzfcBhh96v52odSh6T8N\n\
zRtfr1+elltbD6F8r7ObkNtXczrtsCNErkFPHwdCEyNMy/r0FKTV9542fFufqDzB\n\
hV900jkt/9CE3/uzIHoumxeu5roLrl9TpFLtG8SRAoGBAOyY2rvV/vlSSn0CVUlv\n\
VW5SL4SjK7OGYrNU0mNS2uOIdqDvixWl0xgUcndex6MEH54ZYrUbG57D8rUy+UzB\n\
qusMJn3UX0pRXKRFBnBEp1bA1CIUdp7YY1CJkNPiv4GVkjFBhzkaQwsYpVMfORpf\n\
H0O8h2rfbtMiAP4imHBOGhkpAoGBAIpBVihRnl/Ungs7mKNU8mxW1KrpaTOFJAza\n\
1AwtxL9PAmk4fNTm3Ezt1xYRwz4A58MmwFEC3rt1nG9WnHrzju/PisUr0toGakTJ\n\
c/5umYf4W77xfOZltU9s8MnF/xbKixsX4lg9ojerAby/QM5TjI7t7+5ZneBj5nxe\n\
9Y5L8TvBAoGATUX5QIzFW/QqGoq08hysa+kMVja3TnKW1eWK0uL/8fEYEz2GCbjY\n\
dqfJHHFSlDBD4PF4dP1hG0wJzOZoKnGtHN9DvFbbpaS+NXCkXs9P/ABVmTo9I89n\n\
WvUi+LUp0EQR6zUuRr79jhiyX6i/GTKh9dwD5nyaHwx8qbAOITc78bA=\n\
-----END RSA PRIVATE KEY-----\n\n";

int main(int argc, char** argv)
{
    int ret;
    int writeSz = 0;

    byte out[4096];
    byte tmp[4096];
    int tmpSz = (int) sizeof(tmp);
    word32 outSz = sizeof(out);
    word32 keySz = sizeof(key);
    int algoID = RSAk;
    const byte* curveOID = NULL;
    word32 oidSz = 0;
    FILE* file;
    char fName[] = "./wolfSSL-PKCS8-out.pem";
    char fNameDer[] = "./wolfSSL-PKCS8-out.der";

    XMEMSET(out, 0, outSz);
    XMEMSET(tmp, 0, tmpSz);

    /* convert pem to der */
    ret = wolfSSL_KeyPemToDer(key, (int) keySz, tmp, tmpSz, NULL);
    if (ret <= 0) {
        printf("key pem to der failed with error: %d\n", ret);
        return -1;
    }
    printf("Key pem to der successful, returned: %d\n", ret);
    writeSz = ret;

    ret = wc_CreatePKCS8Key(out, &outSz, tmp, (word32) writeSz, algoID, curveOID, oidSz);
    if (ret < 0) {
        printf("Create PKCS8 Key failed, error returned: %d\n", ret);
        return -1;
    }

    printf("SUCCESS, ret = %d\n", ret);
    writeSz = ret;

/* write DER File */
    file = fopen(fNameDer, "wb");
    if (!file) {
        printf("Failed to open file: %s\n", fNameDer);
        return -1;
    }

    ret = (int) fwrite(out, 1, (size_t) writeSz, file);
    fclose(file);

    if (ret <= 0) {
        printf("Failed to write file\n");
        return -1;
    }

/* CURRENTLY NOT SUPPORTED:
 * NOTES:
 * Works but the header is wrong, by default the "PRIVATEKEY_TYPE"
 * actually results in the header:  "-----BEGIN RSA PRIVATE KEY-----"
 * That is incorrect for this particular use-case, it should be: "-----BEGIN PRIVATE KEY-----"
 * we need a new type RSA_PRIVATEKEY_TYPE and for PRIVATEKEY_TYPE to just be
 * private key for this to be supported.
 */
//
//    ret = wc_DerToPem(out, (word32) writeSz, tmp, (word32) tmpSz, PRIVATEKEY_TYPE);
//    if (ret <= 0) {
//        printf("Der To Pem failed with error: %d\n", ret);
//        return -1;
//    }
//
//    printf("Der to Pem returned success: %d\n", ret);
//    writeSz = ret;
//
/* write PEM File */
//    file = fopen(fName, "wb");
//    if (!file) {
//        printf("Failed to open file: %s\n", fName);
//        return -1;
//    }
//
//    ret = (int) fwrite(tmp, 1, (size_t) writeSz, file);
//    fclose(file);
//
//    if (ret <= 0) {
//        printf("Failed to write file\n");
//        return -1;
//    }

    return 0;

}