Not just necessarily in the show, but in the EU. Can the Borg assimilate the Crystalline Entity? What would the outcome be?
Unlikely in the short-term.
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, 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.
Although there's no real answer in the TV series (or EU books), there is one instance where the Crytalline Entity encounters the Borg and that's in the game mod for "Star Trek Armada : 3"
Due to a bug, the Borg were able to both assimilate and replicate the Crystalline Entity, creating a Borg/Entity hybrid that was essentially unstoppable.