Such an app would be great, no doubt. The folder structure in RC9 differs from that of ADP1,1H and RC33 for some reason though, so anyone trying to create a converter would have to take that into account.
Also, the sizes of the PNG's are of great importance too as the system memory/space very limited. An advanced theme with complex graphics that just barely makes it past the boot screens in RC33 might not make it in ADP or RC9, or vice versa. The different regional images are not identical in that respect, so that will have to be taken into account too.
Though all in all it's definitely doable, for someone with the necessary know-how.
Add a dab of lavender to milk. Leave town with an orange, and pretend you're laughing at it