Hi Amir,
Sorry, yeah looks like that function only exports to ANSI X9.63 format.
I did some farther resarch into possible ways for wolfSSL to convert an ECC key to DER format. We currently don't have a function or process that can convert ECC keys to DER format to then be used in wolfSSL_CTX_use_PrivateKey_buffer(). Thanks for bringing it to our attention though, it's now been added to the feature list for looking into adding a future ECC to DER function.

Regards,
Jacob

Hello Amir
Let me know if this is what you are looking for http://www.yassl.com/forums/topic518-co … o-der.html
If not could you tell us some more about the project and the use case?
Thanks,
Jacob