You can softmod your ps2 to do what you want it to do.
You need to install fmcb in your memory card. Everytime the memory card that has fmcb installed is inserted in the ps2 the ps2 is modded. And you can run your own homebrew (applications) in it.
As for making the usb drive as a ps2 memory card, you can do it by using vmc (Virtual Memory Card) .
You can run games off the usb or internal harddrive (if your ps2 supports one).
And watch movies in it through SMS (Simple Media System).
But first you need to install fmcb on your memory card.
I hope this helps.