add a namned group of secp521r1 to an ec key

ecconv.c

#include <stdio.h>
#include <openssl/ec.h>
#include <openssl/pem.h>

int main(int argc, char **argv) {
  if(argc != 2) {
    printf("give key as arg\n");
    exit(1);
  }

  FILE *key = fopen(argv[1], "r");

  EVP_PKEY *pkey = PEM_read_PrivateKey(key, NULL, NULL, NULL);
  EC_KEY *eckey = EVP_PKEY_get1_EC_KEY(pkey);

  EC_GROUP *ngroup = EC_GROUP_new_by_curve_name(NID_secp521r1);
  EC_GROUP_set_asn1_flag(ngroup, 1);

  EC_KEY_set_group(eckey, ngroup);

  PEM_write_PrivateKey(stdout, pkey, NULL, NULL, 0, NULL, NULL);
}

compile with something like:

$ gcc eccconv.c -lcrypto