Despite the GM's protest about cap space, Tampa Bay clearly has the ability to add. They've shown a willingness to add LTIR players (Seabrook) to manufacturer cap space, and beyond that, they've can use a 3rd team to retain. Now I could buy a reluctance to give up premium assets though. They have virtually no depth outside of their Top 6 though. Perry has been putting up decent points though.
I think Radulov actually makes quite a bit of sense for them because his acquisition cost will be low.
Even if the trade isn't for a Dallas roster player, Ben Bishop actually makes quite a bit of sense for Tampa Bay. Dallas can only move Bishop if they go full sale since they wouldn't require LTIR any longer. They're already committed to 2 more years of LTIR with Seabrook. Tampa is already a contender and potential favorite not making any moves, but adding Bishop gives them an opportunity to load up some value trades (Radulov) or even a bigger fish (Pavleski).