Port models frome L4D2 to Black Mesa

Hello , help me with modding. I want to make a mod that replaces mp5 model to another weapon and for that i want to use the model of l4d2 .Tell me how to port model , make animation and what programs i needed .
Here is a mdoel http://steamcommunity.com/sharedfiles/filedetails/?id=122878115&searchtext=M41A .I think that someone remember old mod with alien"s weapon for hl1

That’s a hella tall order, there, since you have no model creation experience.
You’d need to copy over the materials, decompile the model and the one you want to replace, fix any issues decompiling causes (probably not many if any) rig it up to BM’s arms, create all animations, then compile for BM, overwriting the original model, and hope it works.
It’s kind of a long process, and since you have no modeling or animation experience it might not really be the best way to jump in, but hey, I’m not gonna stop you. Pretty much everything you need to know should be over here. I’d recommend working w/ Blender since it’s small and free, but it doesn’t have proper smoothing groups, so you may have some weird visual abnormalities. Also Crowbar for compiling/decompiling because it’s pretty much the best tool for that, imo.

I have MDL file but I can’t do anything with it

You’re attempting to do something which requires some fairly advanced modelling knowledge, and is more complex than it may seem. If you don’t even know what to do with the MDL file, we can’t really help you, because it requires you to learn all of Source’s modelling pipeline.

As Crypt said, you need to start by decompiling the MDL. You can use Crowbar for that.

I suggest you look up tutorials and just experiment and play with it yourself. This kind of stuff is quite hard to spoon feed, the best way is to learn by doing. Crypt’s been about as comprehensive as possible in his reply. Modding is built on just…trying things!

I already have model and arms skeleton . How to coonect it ?

You’ll need to rig it using weight painting.