You could do this in a cfg as the cvars for this are already in game:
Code:
sm_cvar survivor_allow_crawling 0 //default 0
sm_cvar survivor_crawl_speed 15 //default 15
sm_cvar survivor_crouch_speed 75 //default 75
sm_cvar survivor_damage_speed_factor 0 //default 0
sm_cvar survivor_drag_continuous_fire 1 //default 1
sm_cvar survivor_drag_speed_multiplier 0 //default 0
sm_cvar z_survivor_respawn_health 50 //default 50
sm_cvar survivor_drag_type 2
//0: no dragging, 1: short tug, 2: continuous drag
//sm_cvar survivor_revive_after_drag 0 //default 0
//Standing still after dragging someone will start reviving him
I'm using survivor crawling instead of dragging, haven't tested dragging out yet on l4d2 because didn't work very well in l4d1.