I am not recommending you do what I have described below because you will be doing something with the wrong machine and the results could be deadly so remember you will be at your own risk, however with care and patience what you want to do can be done.
Endmills come with many different style ends on them. You have available balls, radius, cones, flatends with both center and non center cutting ability plus unlimited custom configurations. Plus different materials, HSS, Cobalt, Carbide.
A standard over the counter center cutting HSS mill has the same abilities as a drill in that it allows you to plunge down into your work. Use this mill much like a drill and first plunge down and thru your work piece then raise up out of the hole you cut and move your work piece over with your vice about half the diameter of your mill and plunge [drill] again. Do this same procedure until you have your slot length established. If need be you can lower down into the slot you have now established and do a very slow clean up pass using your vise.
Couple words of caution, make sure your vice 'ways' are quite tight and will not move freely without being manually moved, use High Speed Steel only endmills, and wear a good approved safety glasses.
O.J.