I can think of no way to use random as you would without hacking out the dll, and I don't recommend it. However if you have a few specific bot teams that you prefer you can set up a bind key cycle (similar to a map cycle). That way you can have a limited "pseudo-random" team.
SETUP:
First you need to create a number of cfg files containing the specific (no random) addbot commands for the different pairs of teams. Give these files simple file names like bots_no_fg42.cfg, bots_all_stens.cfg etc. These files then go in the same directory for generic cfg files in RTCW
(sorry I don't play the RTCW version so I don't have the exact directory names memorized.).
Next you need to create a number of vstrings roughly of the form;
Code:
set <vstr_id> "pause;kick allbots;exec <cfg_file>;unpause;bind <key> vstr <vstr_id2>"
Where <vstr_id>, <vstr_id2>... are the names for the variable strings e.g. b1, b2... or maybe b1nofg42, b2allsten... etc.
And where <cfg_file> is a filename.cfg for one of the files containing the addbot commands. (the .cfg extension is required).
And where <key> is the key on the keyboard you choose to use to trigger the change in team makeup.
NOTE: The last vstr definition must bind the key to the first vstr_id.
You should put these set vstr definitions either in a separate cfg file or directly in one of the existing cfg files that are run when you play RTCW. (default.cfg etc). If you use a separate file then you can "exec" it from the command line or from inside default.cfg (or server.cfg) accordingly.
USAGE:
Load the map and pause the bots. Then hit the bound key once or twice and wait for the new bots to finish joining. Then unpause the bots and frag away.
This concept is a lot to put in a post so I could write up a wiki article with example cfg files if you are interested.