Editing WADs that other people have made for such selfish purposes as savescumming isn't a good idea, as it essentially creates multiple versions of the same thing. For example, see this possible chain of events:
1) I modify rosy.wad to change the MainCFG because I'm tired of seeing Rosy has completed Act 1, and instead change her skin name to Amy.
2) I have fun, forget I modified it, and try to join a netgame.
3) That netgame has a conflicting version of rosy.wad (possibly the real one or possibly the one you modified to have savedata!)
4) The md5 sums of the two files do not match, and I am forcefully ejected from the game. I then go off to sulk in a corner because my favourite servers don't like me anymore.
Alternatively:
2) I have fun, forget I modified it, and host a netgame with it loaded.
3) Some hapless souls who have not already downloaded rosy.wad from the MB download it from me.
4) Some join netgames with the real rosy.wad and get kicked.
5) Some host their own netgame with my modified rosy.wad and spread it to new people.
6) Fairly soon, the ratio of real rosy.wad files and clone rosy.wad files is about two parts real to one part knockoff, making it difficult to join any netgame because rosy.wad is so popular.