The problem is that AFAIK, all pillar drills are designed with their bearings to take UP and DOWN forces. Milling presents a SIDE to SIDE and/or BACK to FRONT forces.
I (and many others) do use a pillar drill with sanding drums which present similar forces, and the better pillar drills (with bigger bearings) do seem to stand up to that OK if used with discretion. But we're talking sanding/shaping wood or plastics.
In my case I also rigged up a jig whereby a ball race is embedded on the drill table and the sanding drum has a "nipple" at the bottom which engages with the ball race fixed in a wooden "plate" bolted to the drill table, thereby giving some sideways & front to back support. I hope that's clear?
Of course milling metal (I presume that's what you want to do) will present much greater sideways forces, and it's hard to see how to make up the lower ball race support that I describe above. However, with VERY light cuts, on softer metals (ali?) should be OK, again used with great discretion, and on a one off "emergency" basis.
But although I'm sure that people like Arc Eurotrade (usual disclaimers) can fix you up with a suitable X-Y table vice, personally I would NOT do that, as you'll then be "milling" all the time and I'm pretty sure you'll end up with a pillar drill with no accuracy/lots of wobble because of sloppy quill bearings.
If you can't afford a mill (and I can't, not even a little Chinese jobbie) a much better bet would be a milling "attachment" for a lathe, or even files and drills working by hand can often get you good results.
Sorry to be a nay-sayer but IMHO, NOT a good idea.