As much as I hated losing her as a thief, I hope this is dealt with. I suppose they could make it mandatory (as it should be) by having some sort of quest scene "destroy" Imoen and replace her with a dual-classed Imoen. No way to duplicate the thieving stats you have given her, as far as I know, though.
Perhaps Dynaheir is killed and everyone is afraid Minsc will go into a berserker rage if he finds out, so Imoen learns magic and casts an illusion on herself to look like Dynaheir, but Boo is not so easily fooled.
She also has to get her Belt of Plot Protection, but maybe that'll just be an unexplained gift from Irenicus.