There have been times when I've chucked an end mill in a drill press and done some simple non critical slotting in light gauge mild steel, never been my first choice to do so but when you have your mill running a set up and just need a simple slot for a bracket. As long as like has been said a little common seance and a lot of patience it can be done, never with any tight tolerance expectations though. Plus I find it is always a good idea to have a sacrificial piece under the part you are trying to slot then just make shallow cuts back and forth until you have (I won't say milled) cut the slot through the part The end mill always has a surface under it.
Like has been said if the inside taper is not clean and free of as he called it chowder and the proper matching taper is not used expect disaster, or at the very least poorer performance.
That being said I used to have an old 12" column 5 ft arm fixed height radial arm drill with a raising table I never knew the make or brand of the old machine but it had a 5" diameter quill with a #5 morse and 6" of travel a 26 by 60 table which could be rotated out of the way. My wife drilled millions of holes with it and I very often used it as a vertical boring machine to line bore the bearing locations in aluminum Mack truck transmissions that I later made sleeves to to bring the bores back to factory specs, With the 5 ft long 2,5" boring bar and a bearing mounted on the side of the table and one mounted to the base with a counter weighted leaver to assist in holding the weight of the bar all of the bearing locations could be bored in just about half a day including the set up and location change time. The local Mack dealership would send me about 1 or 2 a week for about a year until corporate realized they had a problem with their transmissions and changed the design. Apparently the the fleet the dealership was servicing wasn't the only ones having that problem.
This is not the old drill press but it looked a lot like it
this is a Fosdick 11 almost identical except for the gear head