sm_cvar survivor_allow_crawling 1
The speed can be set to what you want but anything above 25 is not recommended as the animation gets flakey.
You will need to have SourceMod installed to be able to use the
If you want this running all the time you can add the commands to server.cfg.
All credit goes to Apex: http://www.apex-games.net/?page_id=145
Permission to post this video is here.