PNG and PSD files can't just be converted to OFM files. They will need to be digitized. An OFM file is the Melco embroidery file that has been digitized with the Design Shop software. You will need to find someone who can digitize your files for you and they will probably charge you digitizing fees for doing this.
This is exactly the current need. I've been quoted $100 for digitizing the elephant, so I thought I'd check to see if anyone on -advocacy already had one since I can't be the first to want embroidered shirts. :)
I'll probably end up just paying the money and then give the resulting OMF to the community (assuming I'm allowed to. I'm looking into this)