Well I don't own a copy of WHFRP (at least not yet) so I can't look up how the system is written, but I't imagine it can't be too hard to port things from one game to another...
WHFRP is a d10 based game as I recall... everything is rolled on a d10 (10% per 1 point increment) or a d100 1% per 1 point increment). True20 is of course a d20 based system (5% per 1 point increment).
So bonuses to any d10 roll in WHFRP would be doubled to get the true20 bonus, and bonuses to a d100 roll would be divided by five to get the true20 bonus.
If direct conversion is too complicated, or infeasable, you could just make a system where anytime a true20 character suffers a "disabled" condition from lethal damage they must make a second toughness save (vs the DC of the blow they just recieved). If this save is failed, roll on the follofwing table:
1-3 right leg severed/destroyed, reduced to half movement & can't run.
4-6 left leg severed/destroyed, reduced to half movement & cant run.
7-8 right arm severed/destroyed.
9 right hand severed/destroyed.
10-11 left arm severed/destroyed.
12 left hand severed/destroyed.
13-15 Torso shatered, 1 point of con and character is now considered dying.
16-18 head damaged, loose 1 point of Int, and character is now considered dying.
19-20 - eye destroyed, loose 1 point of cha. (-2 to ranged attacks when reduced to 1 eye, permanently blinded once all eyes are lost.)
Thats just off the top of my head, I'd have to playtest it to see how it works in actuality.