I never played UT 2004 until i picked it up from gog.com and i wanted to know if there was mod or something to let me use MP chars in SP.

I'm not sure I understand what you mean. The single player "campaign" mode has those teams to fit in with the very loose story and the single player instant action mode which is exactly the same as the multiplayer just against bots has all the MP characters avaliable as bots.
If its the instant action you meant then just change the game rules from "bot mode use map defaults" to "bot mode use bot roster" and you can pick and choose the bots you want in your matches.

I'm talking about campaign. You don't get to pick from the whole roster. Like I can't pick any of the robots as my char, or team members.

I know of one that changes the SP campaign up quite a bit Soul's Fury. It lets you choose your own teams and all that plus adds 100+ new maps and a leveling system. Don't know if its more then you want but its very cool.
Explains what it is and does.
[url=]http://forums.epicgames.com/showthread.php?t=552290[/url]
Official site.
http://neeland.king1net.com/Downloads.htm#Soul_Fury_V20

You can pick up most characters as team members by challenging their team to a deathmatch with the team member you want as the prize. Only the team captains are off limits, so you still won't be able to recruit Xan or Brock this way.

Ok that helps alot! Now i know i can get some of others i want. is there any to play as the other char's that don't show at the start tho? Like Is there a way to play as Xan?

Hahaha Epic! Thank you so much for the help! I got Xan in SP and I can get others by winning them. Thanks all!!