Creating signature files for IDA is a 2-step process:
- Create a PAT file
- Compile the PAT file to a SIG file
The tools that come with IDA's FLAIR toolset allow you to automatically generate PAT files for OMF, COFF, ELF, PSX, and Trimedia libraries. The toolset also includes the PAT-to-SIG compiler.
Even though you don't have the library binaries (and thus can't automatically generate the PAT files), given the fact that the PAT file format is documented in the FLAIR toolset's pat.txt file, you can write your own tool to generate a PAT file or even create the PAT file manually (and then compile it to create your signatures file).
However, since the patterns are based on the compiled bytes, you'd still need a binary form of the library code, so you'd still might as well use IDB2PAT or IDB2SIG. In other words, if you can't compile the source code and can't (or don't want to) identify the library functions in your IDB, then you're not going to be able to create traditional IDA signature files for that library code.