This is something we should support for MVP, although not in its full incarnation necessarily.
We must have the ability to mark specific posts as being under a license distinct from other content on the site; this could be as simple as having LicenseName and LicenseLink fields on the post model that are null unless the content is differently licensed. However we do it, the ability must be there, because that lets us specify (in a configurable item) which license to use for posted content, while still being able to import SE content under a different license.
What we don’t have to do (and probably shouldn’t) for MVP is to provide the ability for users to choose the license for their own posts - there’s a discussion to be had about the merits of that, and it’s a bunch more development work that’s not essential to launch.