The MDL files are better described as ‘info’ files. LUA ‘info’ files. You can edit them just like you can edit any other ‘lua’ file. The function of mdl files is to grab the needed resources (meshes, sounds) and give the whole package a name and description (and pricing and other info) to form a ‘model’. The meshes and sound files are stored separately. In this file you can edit the info / running costs / life time years and so on
So that’s why you can’t import them into blender because blender does not know what to do with it.
(by the way, it is ‘not’ good that people can not import models into blender because nobody can ‘learn’ how to make their own mods for the game. With a long wanted lifetime for this game modding is a ‘must’. That ‘i do not like others to open my files’ is just crap. Even if you can open them you are just not allowed to ‘use’ them in your own sets because the game is protected by copyright laws. But viewing to learn should be no problem. So that’s already handled.. don’t worry about that.)
My tip to the game developers is: release a simple mod tool (or just release the tools you used to compose models (packing them, put them in the right directories, the descriptor files (mdl files)), i would be amazed if you didn’t use a tool. I mean.. hand-editing all these files is just time-consuming and boring and error prone)