S2General/Converting Source 1 Models to Source 2
Source 2 has a built-in system for taking old .mdl files from Source 1 and converting them to Source 2. It's not a perfect system and 9 times out of 10, it will not compile, but it can be great if you are porting content en masse from Source 1 to Source 2.
Supported MDL Formats
This system is very picky as to what kind of .mdl formats are imported. Currently, Source Filmmaker so far has been the most successful place to port models from, which can be beneficial as the SFM already ships with content from a ton of games. However, Half-Life 2 and Left 4 Dead 2, among others, have all been tested and are incompatible, but luckily, Valve has already ported a lot of content from those games.
Creating the .vmdl File
To convert the .mdl file, you must first create a structure for your content to live in. Lets say I'm converting a model from Half-Life 2, lets say "models/humans/group01/male_07.mdl" for use in Half-Life: Alyx. To do this, you will need to have the tools installed and a content folder for the mod you're attempting to compile for. In this case, I have a mod called hl2 which has a corresponding folder in my content and game folders inside of "steamapps/content/Half-Life Alyx".
Next, you'll want to drop your .mdl and it's corresponding files into the same file structure it had in Source 1. So for instance, my male_07 model files will now be in "Half-Life Alyx/content/hl2/models/humans/group01/male_07.mdl".
Finally, you'll want to create a new .vmdl file. Open it in your favorite text editor and add the following code to it. Close the file and then open the Source 2 Tools.
<!-- kv3 encoding:text:version{e21c7f3c-8a33-41c5-9977-a76d3a32aa0d} format:generic:version{7412167c-06e9-4698-aff2-e63eb59037e7} -->
{
m_sMDLFilename = "models/humans/group01/male_07.mdl"
}Once done, you can now open your .vmdl file and it will automatically compile itself to a .vmdl_c file!
Other Notes/Broken Things
The biggest problem this auto-conversion tool has is that flexes are non-functional. Some have their flexes vertex data completely destroyed (such as HWM models from SFM,) some just have a mix of working/non-functioning flexes.
The other problem is the lack of support for certain Source 1 Engine branches, so importing models that haven't already been converted for Source Filmmaker can be quite difficult.