Most people would probably not call this process "converting", but it really is just another conversion similar to the other ones in this section. It is commonly called "decompiling", because it is changing the binary files into text files.
NOTE: Instead of doing the following nonsense using the outdated MDL Decompiler, you could use Crowbar.
Place the extracted L4D2 survivor files or the survivor addon files that you want to convert into their own folder. There should be a vtx file, a mdl file, a phy file and a vvd file.
Verify two files exist, one with extension of ".dx80.vtx" and one with ".dx90.vtx". If needed, you can copy an existing vtx file (".vtx", ".sw.vtx", ".dx80.vtx", or ".dx90.vtx") and change the copy's extension.
Open the mdl file in Notepad++. (The first four characters should be "IDST" [without the quotes]. The fifth character is usually a zero "0" [without the quotes], but can be one of several different characters.)
Change the fifth character to a comma ",".
Save (overwriting same file).
Close Notepad++.
IMPORTANT: Because of bugs in MDL Decompiler, ALWAYS restart MDL Decompiler for each decompile, whether it fails or succeeds. Not doing so will likely cause problems with the next decompile.
Open MDL Decompiler.
Uncheck the "Use Steam File Access" option at the bottom.
In the "Choose Model file" option, select mdl file.
In the "output" option, select the folder that the mdl file is in. (MDL Decompiler should default to this folder).
Click the "Extract" button.
Close the couple of windows that pop up indicating error or success.
Close MDL Decompiler.
Verify that there are new files in the folder: one or more smd files and a "mdldecompiler.qc" file.