if you are talking about just using that 15.5 gallon keg to boil in, it is definitely possible, and easy to do, in fact. they are referred to as a 'keggle'. i have two, one for my HLT, and the other for my boil kettle, and perhaps in the future, another one as my MLT.
all you need is something to cut the top out with: dremel, saws all, angle grinder, etc. and something to cut out a 7/8'' hole for a weldless fitting (hole saw or a uni-bit), or someone to weld in a coupler for you. then you add your ball valve (i prefer stainless), and your nipple (i prefer stainless again) and you are set to boil in it once it is clean, and drain with the spigot.
if you are talking about fermenting though, there are some people who ferment in the sanke kegs, but it takes a decent amount of work to make it happen. at that point i'd recommend getting some USplastics drums, maybe their 14 gallon food grade with removable lid, and use that to ferment in.