I am not saying they would have a hard time adding someone to the 40, I just don't think they are likely to do it between now and the trade deadline when all these things will work themselves out. There is nothing Wilson or Stevenson could do in the next three weeks that would convince them not to trade for better OF bat. They come up and hit .500 and they are still just Clemens or Sosa or Dahl getting hot for a while and we already have those guys (probably even Dahl still).
I also don't think they are prepared to DFA Pache or Merrifield until that all plays out. Even Merrifield could end up in a trade under the right circumstances (like, because it was mentioned - if the Phillies were to trade for Suzuki, he makes enough money that they could make the Cubs take Merrifield back, even if the Cubs didn't keep him).
The front office and clubhouse are also still a little blinded by Merrifield. Dombrowski has to admit it was a mistake, and Thomson (and Harper and other players no doubt) have to admit his grit and pedigree and character mean nothing weighted against his performance. It's coming, just not this week.
Pache is presumably easy to include in a trade once you remove the idea that he's Rojas insurance or someone who could still be on the team next year. But if they were to trade for a CF with Rojas in the deal he'd probably stay.