In many programs there is the possibility to expand the keyboard shortcuts.
Usually, two combined key sequences will give one command:
Ctrl + U + [user defined shortcut]
The prefix Ctrl + U is followed by any key combination as usual:
Ctrl + U + F5
Ctrl + U + Ctrl + X
Ctrl + U + Ctrl + S
That way, a lot more keyboard shortcuts would be available.
To keep the complexity low, only one specific key combination [Ctrl + U in the above example] should be allowed.