Png texture file

Hi Guys…I have created a png texture file with several textures on it, and used it in sketchup 2017 to add all my textures to the faces of my model.
Exporting to x-plane then was no problem. However I forgot 1 texture so I added that texture to the png texture file using canva as I always do but now if I try and add that extra texture into my model and try and export it to x-plane I get an error saying I have used multiple texture files ?, even though the extra texture is in the same texture file with all the others, everything is power of 2. is it possible to add an extra texture or two after using the original fle or not. would appreciate anyone that can help. John