Armin Diedering made a mouse wheel driver.
On his page for mwheel he says
I'm open to everything, I MWheel made compatible to XaAES.
so he might be able to help you get it to work with CosmosEX.http://www.ardisoft.de/Produkte/MWheel/mwheel.html
Just send him an email and ask. He is also on atari-home.de as "ardi" (ardisoft). Don't know if he is a member on this forum.
*EDiIT* Yep he is on this forum as "ardi" also, but last visited in 2014 so it is best to contact him via atari-forum.de where he is active or via his webpage contact.