It can be done, but so far it is in MP-only due to how servers and clients work. There is more freedom to do things like this that area, ironically.
First, you have to unlock all the character classes Rocket banned... Which can easily be done with a mod servers use... And then you have to edit both the server itself as well as the dayz_code.pbo, too.
So, like I said, yes and no.
I haven't looked into it for SP since the only other character models would be military (soldiers and support), or civilians. Lingor used the rest of the PMC addon characters, but again, this was a server + modified client because not everybody has the PMC DLC with those extra models. Everybody who owns OA has PMC lite as part of patch 1.62. These are the low-res versions of the PMC models (Survivor variants) Rocket used to make sure the game was playable by all ArmA 2 owners and not just PMC owners.
I stumbled upon a mod or something, wich had an assload of hightexture ghillie suits, wich camoflaged into many object. That was pretty dope. Let me see if I can find it. It's probably for server owners, though. :/