Sorry, I’ve searched for the answer … and I’m tired of searching, I wanna do something productive 
I made a project template which took a long time, now I wanna assign wav-files from sources list to he empty tracks of the project …
Can someone tell me if and how this can be done ?
Thanks
Drag and drop it.
(If you mean the source list in Ardour, from the right pane in the edit window.)
-What @Piergi said.
This:
Note: You can get to the Source List by hitting shift-L and then clicking on the “Sources” tab.
…
Edit:
Wait, by “assign” do you mean permanently keep certain regions/sources saved to a project template, so every time you start a new project and use the template the same regions/sources appear on the particular tracks you’ve set?
If so, it doesn’t seem like you can do that via Session → Save Template… (Someone correct me if I’m wrong.)
Instead, your best option would probably be to simply create a project like you have (but with all the sources/regions you want), and then re-save or duplicate (and rename) that exact project, and then edit that/those duplicate(s).
-J