I don't think they make any, if very little after market goodies for the Mossberg 100ATR, that includes the type of thumb hole or pistil grip stocks that you mentioned, that I have ever seen. Some of the other guys may know something though mossy.
"There is no hunting like the hunting of man, and those who have hunted armed men long enough and liked it, never care for anything else thereafter." - Hemingway
The greatest ignorance is to reject something you know nothing about.