add-on episodes?

i don't have a playstation 2 yet but have been thinking of getting one, and this is definitely a newby question but i really hope someone will be so kind as to clue me in.

on a pc computer, in games like Doom or Quake 1/2/3, you can download homemade episodes and unzip them into the proper folder and then play them in the game. these are not of course 'factory-issue' maps or episodes, but 'aftermarket' add-ons.

can this be done with a playstation 2 for these and other games?

i think i realized the probable answer to my own question ^. playstation possibly does not allow people to even make their own add-on episodes compatible with playstation. or do they?