Teaching her to love anal invasion is easy enough to do, with the right tools. Pick a toy she likes (the Hitachi always works, of course), and a variety of gradually widening butt plugs. Start slow and use lots of lube.
Begin by arousing her (something easily achieved if she’s kept in denial, of course). Once she’s wet and squirming, introduce the first and smallest plug. Hold it in front of her, and stimulate her clit. Make her beg you for the plug. Once she begs you, carefully and slowly insert it into her ass. Keep it there and continue to arouse her for a while. Let her get used to it.
Do this over and over until she can take the largest plug you have. By the end, it won’t take her long to beg for it at all. If you let her know that, after X plug is inserted, she can have an orgasm, she will move right along at a good clip. Of course, don’t tell her how long after it’s in she’ll have to wait for that orgasm… just dangle it as a carrot.