I just sent this to some one and realised others may ask so here it is...
This is the way I have done it.
Place valid paa files into a texture pbo.
put it in you addons folder
On loading Wrpedit, load your map....
goto surfaces tab on right.
Click on ADD
then click on ADD again (its very important this one)
find you file with the textures in, ideally in the ADDONS folder
goto down the list and find you pbo name WITHOUT the /
Select your textures in the big area which shows all the individual texture names
Place using these.
Save your map and pbo it with map and cpp.
Thats how I did it, and it works for me.
The reason I do not put the textures in the SAME pbo as the map is that in my map the texture pbo alone is 3128kb.
The map is a lot smaller and it means that I dont have to send a big file over the net when I do a object placment changes to the map, they afterall already have the textures in a seperate pbo..