I put arm rests onto a t4 seat and it was quite easy.
Not sure if its the same on a T5 but if you lift the seat cover up on a T4 you have two tubes running up the side of the seat.
All I did was make a plate that would bridge the two tubes (back to front) at the correct height, I then drilled a hole where the arm needs to bolt on, then welded a nut to the back of the plate (correct size for the capscrew that goes fixes the armrest)
This allows the screw for the armrest to go through the hole you have drilled in the plate, and tighten onto the nut.
Then all you need to do is tack weld the plate to the upright tubes job done.
I did have some photo's showing seat cover off and I will try to find them and post.
As I said the T5 seat may be different but im sure it can still be done fairly easy.
Hope this is of help.