there are other/better forums for playstation 2 than tom's. here when people read "ps2" they think "PS/2" not playstation 2.
regardless, i know that there were adapters that convert PS2 controller port to USB, it's purpose was to use a PS2 controller on PC, but i doubt it could be used on a PS2's USB port. if you want to plug in more than one controller you have use a multi-tap, that's the point of it.
also, the fat PS2 and slim PS2 use different multi-tap adapters. AFAIK the multi-tap for a fat PS2 will not work on a slim PS2.