Is it possible to use a drill press as a milling machine with the proper bit?
The thing that a traditional milling machine does well is handle sideways loads. Your drill press is designed for up/down loads.

Assuming you get the bit speed right, the downside to your plan is premature wear of the bearings on the drill press.

Now, if you can use it to only cut down into the stock, then you should do okay (though slow). (Assuming adequate safety/clamping of stock/etc.)