You most certainly can. Your best bet is the MLB 2004 All-in-one installer, available at Joe Rock's wonderful
www.ootpdownloadcenter.com . That installs ballparks, facepacks, logos and real rosters into the game. I'm pretty sure Eriq said he was working on a 2005 one as we speak.
There are a whole ton of roster sets, images, ballparks etc. at the download center. Have a browse! And welcome to the greatest game ever coded!