First thought is: why?
Second thought is that it would be expensive even if it could be done because: new stock needed? Buddy bottle kit needed? Action bit and bobs needed to fasten buddy bottle?
Third thought: Sell the HW100 and buy a dedicated buddy bottle rifle. It could well work out cheaper.
Fourth thought: I doubt if it could be done????? and produce a good looking / handling rifle.
Cheers, Phil