I made gumpaste from the CK products mix but its too soft and droops when making roses. Can I stiffen it just by working in more powdered sugar? Relatively new to gum paste and have only used wilton ready made - with no problems but gets expensive. Thanks!

You could try that or add Tylose or CMC powder. I add that to my fondant if I need it to firm like gum paste.

Assuming that you have allowed it to mature about 24 hours then, yes, knead in some more powdered sugar. Don't get it too firm; however, as you will have a product that is practically unworkable.

I have found that recipes that say you can use it right away are still better if one waits 24 hours for it to "cure."

Edited to add a warm welcome to the OP!