So, to sum it up to answer your question: yes, you can place the mage (or witch, if you regard this makes sense, which it does not) in the same feature like your new placed meeple, and even on the same tile. You have to place your meeple on the same tile, you placed (like normal), the mage or witch you can place on any tile. The only restriction for them is, that the feature has to be a road or city, and it must still be unfinished. You are even not allowed to place the mage or witch in a feature, you finished with this tile. So e.g. finishing a 4 point city and scoring with a drop in/ drop out meeple is still possible, but you can not extend the scoring by additionally moving the mage to it.