You can just stick the pk3 in JA's base folder and the model will show up and work in-game.
The sounds won't work and you will have to fix that seperately. Information about the fixing the sounds can be found over here:
I'd recommend using Hekx's seperate pk3 method if you'll want to play online, or just for the ease of not repacking every pk3.
You'll need to look in the pk3's to find the name of the sound folder and model folder, and then just create text files with the sound directory and put them in folders with the model directory names. Zip that up and the sounds will work.
I hope that was clear enough. If not, just ask and I'll clarify as best as I can.
EDIT: There are a lot more models and things for JO that will work in JA. Here are some sites with those files (you might already know about them, but I thought I'd post them anyways.). There's a lot of really good work, so if you don't mind searching around you can get a lot more good stuff.