Cant really see why u want to make them freeze, except for if u want to rename them into some good players u cant otherwise kick the ass off and want them set up for some headshot ownage for movies.
Anyway, i can maybe conjure up some mutator or mod code that can zap the bots movement, but its only going to work for offline games.
U should start by modifying the bot definitions in your user ini:
BotSkills[x]=-3.0 // as dumb as possible
BotAccuracy[x]=-1.0 // well if they shoot, u dont want them to hit u, right?
CombatStyle[x]=-0.5 // same as 'cautius' in the ut menus. I.e. not agressive & up ur nose.
Alertness[x]=-1.0 // modifier for awareness radius, frequency and probalility.
Camping[x]=1.0 // want them to stay put.
StrafingAbility[x]=-1.0 // no strafing
BotJumpy[x]=0 // 0=false -> not jumpy.
If that isnt enough, try bribing me to write some code for u