The Insert key without 3rd party apps on MacBook Air in VirtualBox running Linux
It is not what I am looking for but it is a fine solution for the time being. If someone suggests a way to simulate the Insert key on MacBook Air I'll be happy to accept their answer.

A workaround in the Debian Linux itself

If you run xev to see what keycodes are actually being sent when you press keys you'll probably notice that fnEnter produces keycode 104 KP_Enter instead of keycode 36 Return. 

I decided to remap keycode 104 to Insert with xmodmap -e "keycode 104 = Insert".

As a result I am able to paste with fnShiftRight AltEnter.

References


Missing Apple MacBook Keys: How to map the INSERT key in Linux/VirtualBox
xmodmap on ArchWiki
Remap the insert key