The Help says it must stay the same for the whole song.
Unfortunately, that is true.
But you still can mix time signatures within a song. Change the time signature according to the part of the song you are working on, to make the loops with that signature available in the loop browser, then set it to a different value, when you work at a different part of your song.
The only problem will be, that the measures will look all wrong in score view, or when when you try to print the score, but there is no problem to mix loops with different time signatures in one project.