Could I save my own custom XMB theme with my PSP?

I was wondering if this could be possible. I have downloaded several free ptf Custom XMB Themes. And then what I do is choose the theme that has the best looking icons. Then I replace the background image to something in my picture folder. Now I have the icons and picture I like. I was hoping to learn if there is a way to save this custom XMB theme I currently have in my phat PSP so I can later share it?