Although I haven't tried it personally you should be able to run windsor and SM side-by-side.

If you keep the windsor container and you want to inject something EPiServer-specific you'd have to handle those registrations yourself. Dependening on your setup with your current container and the number of registrations you are interested in from EPi's container you could either register those manually or build some windsor plugin that checks itself for dependencies first and delegates to EPi's container if the dependency is not found.