I'm not a crypto-expert, but I'm wondering.Can XCA register with Windows as a security provider? In other words, Windows use certificates stored in XCA database to connect to a VPN server (like smartcard usage).
Not to my knowledge. XCA was not made for that purpose. It was made to create and administer certificates - much like an external CA does - just that you have that function on your PC. And would an external CA per used as a Windows Security provider ? Not Really !