I'm trying to create some application for Windows Phone >= 7.1 During plannin my work and specyfiong technical requirements I've come across one problem. I wan't to have free application with basic functionality. And then it should be extendable with some non-free extensions (dlls?, databases?, SaaS?) Is it possible at the moment to use some MS api/app/etc. to be able to publish such extensions and sell them? What is the best way to do this if there is more than one solution?

Thank you for any help.