Not possible without editing the code because those pants are shared with a lot of units.
It's possible, I managed to separate all of the British, French, and Russian pants in my mod. It takes quite a bit of time and patience though, so grab a nice beer and some good music, and settle down with OpenBRF.
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What you'll want to do first is single out which unit you want to change the pants mesh for, let's take the French as an example, because apparently their trousers were so comfortable everyone decided to wear them. The basic French pants are located in the mm_french_uniforms.brf file, they are called
french_basic_infantry_pants and they are currently linked to all basic French infantry units, and some others in other factions.
I'm going to take a second to explain how the unit meshes are currently set up in the uniform BRFs. For all units, you have the torso meshes and the pants, the torso stuff will all have the same name with a number behind it. The pants are all separate, either with the unit's name in, or shared with a pool of other units. MW looks at the torso meshes and then combines them with the correct pants mesh that it has been told to combine the torso with. If it can't find one of these meshes, it won't load it, and you'll get an invisible body part.
Now back to business....
First thing, rename your French infantry pants something else. I renamed mine "french_basic_infantry_pants (old)", now NW won't load that mesh because it can't find the right name. Load up NW and have a look at which regiments are missing their pants, make a note as this will save time later.
Head back into OpenBRF and copy paste the "french_basic_infantry_pants" mesh underneath all the torso meshes of the units that were missing their pants in-game. Rename the meshes to the name of the torso and add the next number in sequence at the end. Here's an example:
This:
french_84e_body_ranker
french_84e_body_ranker.1
french_84e_body_ranker.2
french_basic_infantry_pants
Becomes:
french_84e_body_ranker
french_84e_body_ranker.1
french_84e_body_ranker.2
french_84e_body_ranker.3
Because you changed the name of the normal French infantry pants, NW won't load them, but because the torso mesh now includes the pants, they'll be loaded with the torso. Great hmm? Simply do this with all the units you listed as not having pants and you're almost there.
To make sure that they don't vanish at certain distances, right click on the mesh and choose the option "Compute LODs". Save the BRF and you're done!
Hope this helps anyone who was wondering about the pants problems. I'll have a look at breaking the shared shako problem as well.