What all is required to produce OMF from ELF? Is there a doc to read?
Here's a program that can do it,
https://www.agner.org/optimize/#objconv along with the documentation and source, it builds easy on OS/2, there's a copy on Hobbes.
I'm sure there is also lots of documentation out there on the OMF and the ELF formats.
As OS4User points out, another possibility is just staying with ELF until the linking stage and having a linker that can link ELF objects into LX executables and DLL's. Wlink can handle both file formats, whether it can currently build a LX executable or a DLL out of ELF obj files needs investigation. It's ELF support isn't the highest as it can't even create ELF shared libraries.
Either route needs someone who knows this stuff.