I had forgotten it only worked with bonus powers, been a while since I last did that. But, what you suggest may work through coalessced modding, as kind of a work-a-round to get class powers into other classes.
I have a total of 15 powers plus unity. The way to do it for me was to have your original 8 in the shift and move to the UI and then using a keybind add other class powers. These powers will replace the ones in shift but keep the UI the same. In my case I have all my other powers keybinded to numpad 1. The only problem was the power wasnt leveled so I went into coalesced and upgraded the base to the leveled up max so they are useful late game.
You can open 2 instances of the save editor (i used Gibbeds) with a different save open in each. Then copy/paste from one to another. I had to do this for my soldier with slam. To get the power available though u need to navigate to the "Raw" Tab, then Squad > Player > Powers (you will click the "..." button to the right of where it says "Collection" then click add and paste the information into the power. To have it available in-game on your power list u need to assign a "Wheel Display Index". Starting out as a soldier from ME2 save game, i had number 7 open. I dont think they are necessarily in order though, for instance #1 isnt the very left hand power i believe. You can rearrange your powers this way also. Hope that helps?!