Unlikely in the short-term. Every
Every time we've seen assimilation occur, it seems to require a being's bloodstream to disseminate the nanobots involved in the assimilation process. It seems unlikely that the same technique would work on what is essentially a really big rock. That
That, plus the neural interfaces the Borg use to connect to the Collective probably require neurons at the very least. A whole host of problems would probably occur, all centered around the issue of Borg tech being designed for humanoid, or at least biological (rather than geological) lifeforms.
That being said, just give them time. The Borg are nothing if not adaptable.