Is it possible to add more logo's to intro
without replacing a logo?

only with very expensive, technical programs
not worth it

Yes: by changing the background itself and not the acctual logos.
You can do it this way:
(i assume you know how to use GFXPAK or EA GRAPH)
1) look for \Data\Gui\Back\back.big
2) extract lback1.big from it
3) extract lback1.fsh from lback1.big
4) edit the background and save the fsh file
5) insert fsh file into lback1.big
6) no need to insert lback1.big to back.big just place it in the same directory

Tal B