But I found a workaround, I didn't have the time to really test though : 
Create a custom action, with an applescript function: (this one increases by step of 1 
" 
set current_volume to output volume of (get volume settings)

if current_volume is less than 100 then 
set current_volume to current_volume + 1 
end if

set volume output volume current_volume 
" 
(do not copy the double quotes, only what's inside) 
You can decrease volume by 1 using : 
" set current_volume to output volume of (get volume settings)

if current_volume is more of 0 then 
set current_volume to current_volume - 1 
end if

set volume output volume current_volume 
"

As I said earlier, I haven't tested it yet (not at home currently), but it might work. But having the possibility to repeat the SHIFT+OPTION+Volume up/down would be easier (and also would bring the possibility to display the volume OSD.

I really hope the RB developpers implement it soon!