Okay, seems there are several ways to do it...
Right clicking on the collect clip gives me the options to save as Motion or MotionPlus to the file system.
Now i realized that i can press '+' when browsing to a iClone custom folder in the content pane.
I don't know what's the difference but in both cases the motions then appear in Clone and i can use them!
Thanks for pointing me into the right direction