If it doesn't fit, get a toy that does, get her used to that, then work up to bigger toys that are the size of your dick. Use lube always. This is a MUCH more effective, safe, and enjoyable option. Putting coke in an ass that physically can't fit a cock makes no fucking sense. Come on, think about it. Besides, anal training on your girl is fun. I have nothing against getting high on coke and having anal sex, but don't use it to numb the fact that it won't fit.
@ Da Bomb: Be aware that plugging cocaine makes for a much faster onset and intense effects. You said you don't normally do drugs like that, so it sounds like you are jumping into this idea without much info, please be careful. And like I said, use toys over a period of days/weeks/however long it takes to actually get to a point where you can comfortably fit it.