Well, the WrpTool manual is a good place to start. It shows a picture of how transitions are handled. The gist of it is that each transition texture can be visualised as being cut into quarters, with each quarter representing one of the four surrounding terrain types.
ptptb1s2.pac is beach,beach,forest,stone
ptb1s2pt.pac is beach,forest,stone,beach
b1s2ptpt.pac is forest,stone,beach,beach
s2ptptb1.pac is stone,beach,beach,forest
If you examine each of these textures in texview, you will see they are all the same texture rotated by 0,90,180 and 270 degrees(warning texview shows textures upsidedown when compared to wrptool). Therefore if you have just one of the above textures you can easily make any of the remaining three. In texview open the texture you have and then save as, select any file, and give it tga extension. Move the tga to a temporary folder and use your fav graphics editor to open and and rotate it as required. Save the new tga(24 bits) and name it as the texture you are missing. Now use texview to convert it to a pac. You will have to make sure the texture has the right filename, texview adds a paa extension, not pac. Now place the new texture in the folder of the island you are working on. In wrptool import the new texture. That is all!
edit: I forgot to mention that I had to make one texture myself by blending two together in a graphics editor.