IFF you want to support the default set of keys installed on computers that ship with Windows. Secure Boot does not prevent you from installing your own keys, in fact most linux distributions do this already and just use a shim loader signed by Microsoft, the rest of the chain is signed by custom keys (the keys are silently and automatically installed for you).
IIRC, Secure Boot spec said there must be multiple trust anchors, i.e. it's not like "user's own or Microsoft", but there can be any combination of trusted CAs (and I bet there's NSAKEY somewhere, huh).
I'm not sure about the implementations and real-world situation, but as far as I get it, with X.509 with Secure Boot generally uses, one should be able put the exact card's vendor certificate (not MS CA root one) to trust the extension card. (Sadly, I think there's no way to trust one specific signature.) I guess that's probably very non-trivial in practice.
At worst, one should be able to put their own CA (to sign their own software) and be forced to add MS CA to trust the third-party software as well. But - if UEFI implementation allows user-defined CAs - it should be possible to run your own code without asking Microsoft's permission.
