Well, It's not hard. You just need to know how to edit files of course.
First step:
You need an two folders with models:
First model: An original survivor (for physics)
Second model: Custom model.

YOU NEED TO KNOW! You need to change the $modelname "survivors/survivor_manager.mdl" to "l4dport/yourmodelname.mdl" (leave the $modelname)

Open Crowbar, make sure that all folders lead up to Garry's Mod launcher and tools folders (If no, set up) then go to decompile click browse and choose the original survivor model. Then click decompile and done, also do it for second model. Then go to original survivor model. You need to rename survivor_manager (for example) *folders with anims, .qc, .vrd, .vta, and .smd (smd it's either physics or lods)* to yourmodelname. Then go to qc and replace all survivor_manager to yourmodelname. Now go to Crowbar click compile then browse and choose your QC. Now compile it in another folder (also click browse and seek for folders). Now you have original survivor model with different name and physics . Now delete mdl, vvd, and dx files (.phy don't touch and don't delete). Then go to decompiled custom model and do everything as you do with original survivor model (rename the qc etc.). Compile it in the same folder that .phy located. And.. Here we go! You have an model that was ported from L4D2 workshop!

Questions and Answers:
Q: What about PM anims? How to make it?
A: Well you also need to know that all anims are down in the qc file. If you have an L4D2 anims (that named like Manager_ not like $sequence) you need to delete it. Then if you have like $includemodel "manager_anm.mdl" you need to replace it with $includemodel "f_anm.mdl" ($includemodel "m_anm.mdl") And you need to know that you don't use Blender software. So bones will be broke. That why I always do the ragdoll.

Q: Bodygroups? How to make?
A: Where
}
$bodygroup "top"
{
studio "coolshirt.smd"
}
You need to add one small word at the bottom (also you can at top) of studio: blank
}
$bodygroup "top"
{
studio "coolshirt.smd"
blank
}
That how bodygroups works.