This may work for you [a similar thing happens to me]:
I find I cannot reuse a .pac or .paa, even though it was O2 that created it for me. If I want to map a texture, I must import a TGA.
So, even if I already have a basic_hull.paa, I still import the basic_hull.TGA that it came from ... and O2 accepts it, lets me use the image or map it to faces, etc.
It's up to me to remember to CTRL-doubleclick this "new" texture in O2's textures window, click once somewhere on the dead space of O2's interface [like the area between button groups], then press e. From there, I must change the path to reflect the actual .pac or .paa. in its proper path.