This is already working with a little trick since several years:
Go to ->Search ->Book Search ->Advanced
tick 'My Shelf Only'
And search for the title of the book, where you like to add a cover.
In the results you will find under the book information now following functions displayed:
edit book details | New Journal Entry | make release notes | update cover
Choose 'update cover' and load up your picture/scan.
Oh, I got this to work, thank you!
This works only while the book is in your hands.
Wish I'd known about it sooner...but I'll try to do it from now on. :)