On Doctor Who, they managed to get the entire cast from all three shows (Who, Torchwood, and Sarah Jane Adventures) together for the season 4 finale.
Crossovers can be done. I always thought it was a budgetary issue. They'd have to pay two entire casts to appear in the same episode.
I think Sito Jaxa would make sense, for a DS9 crossover. Another feisty Bajoran, like Ro or Kira. I liked the character, and think she would have fit in on the darker DS9