Hi there,
I'm on mobile so please forgive the lack of thoroughness.
Adding .PBO's from ArmA 2 is a matter of copying and pasting them into the "Addons" folder or your own @ModName folder in the DayZ directory. In order to configure weapons to work with the new DayZ system, you will need to compare the old config.cpp from the ArmA weapon you're converting with a new config.cpp from Standalone and make changes as you see fit - this requires a lot of trial and error to get right.
In doing this successfully, you will be able to spawn items using a server script however 99% of the time you will have to modify the P3D in order for the rifle to line up correctly in the players hands/iron sights. This requires unbinarization of the P3D (guides can be found on this forum for doing this) to edit, then rebinarization to use it in game. If you want new attachments from DayZ SA to work with the old weapons, this process is also essential as DayZ uses a system called proxies to define where the attachments will be located on the gun. In order to add proxies, you must load the unbinarized model into ObjectBuilder.
Because of the completely different clothing system in ArmA vs DayZ, I have not tried to port clothing from ArmA 2 over however if you use the same principles as weapon porting, it should work fine with some editing of the P3D's.
Most other items should work fine following the same guide as the weapon one.
Let me know if this works for you.
Cheers,
Uhneb