Not by the rules, you can't - well, unless the creature is 2 sizes bigger than the ally (well, unless it is much bigger/smaller than the ally, or helpless). PH, p. 286, first bullet of page - you can only force move a model into a space it could enter by walking. Therefore, the rules on p. 283 apply, which state when you can (and cannot) enter an enemy space.
That said, there is some debate on whether one can "choose" to "switch off" being an enemy. My own opinion is that you cannot (barring such situations as actually becoming an enemy to your allies, aka P v P) - the rules really aren't designed for such, and allowing it opens up the game to way too much munchkinism.