I've seen several model 99's with full stocks.
I also have one of the NRA muskets listed above. As stated, the handguard is not fat. It is the same thickness as a standard model 94.
I've never attempted to have a lever converted to full stock, but have done some bolt rifles.
Making one, or having one made is a different story. Most smiths will shy away these days due to the amount of work that goes into a full stock rifle. Most all will comment, "why do you want that, it won't be accurate".
I'll agree, most will not be as accurate as a top of the line custom made to shoot tiny groups. But I have 3 or 4 full stock toys that will shoot better than I can. And only one that is shabby (1.75" plus)