I would call Pella, and ask them. They will be the best since they know what plastics the window is made with., and how much the windows move when it gets hot.
Let use know what they say, it may help others.
I would also like to find out what Pella says. I'm going to go out on a limb and guess Pella will not recommend any glue on their windows for fear of ruining the expansion ability, but it could probably be done.