Yeah, P07 magazines won't lock up (too short) in a P09 frame. I suppose someone makes extended magazines for the SIG pistols so those might be long enough to work.
One of the members here, couple years ago I think, shortened the frame on his P09 to P07 dimensions. You could search for the thread to see some pictures and description of how he did that. A P09 with a P07 length magazine well might also do the trick for you.