How do you add a subjectNameAlt extension to X509_REQ?

I am creating a CSR which is going to be processed by my server. It needs to set the subjectNameAlt so that the server can process it. I've searched far and wide, and have only found how to do it with normal X509 certs, not X509_REQ. How can I do this (with C and OpenSSL. I.e. I need the equivalent of X509_get_ext_d2i but for X509_REQ)?