I'm looking at the sspiprovider class, and I see that the methods are all imported from crypt32.dll or advapi32.dll
Would it be possible to write something like a dropin replacement for that class that uses the OpenSSL dll files for encryption instead.
If would solve the problem with glftpd2.0, since we know that libeay32.dll and ssleay.dll support the proper cihper suites, since they are used with programs such as flashfxp (delphi) and ftprush/ultrafxp (unknown language)

No, replacing the CryptoAPI calls with OpenSSL calls is not possible (unless you want to rewrite the entire library).