If you want to convert your peptide search results into something that Skyline can understand, you might want to convert it to the "spectrum sequence list" (.ssl) format:
https://skyline.ms/wiki/home/software/BiblioSpec/page.view?name=BiblioSpec%20input%20and%20output%20file%20formats
You must have at least one decimal digit of precision in your mass modifications, but you can have more if you want, especially if those extra digits of precision will disambiguate between your modifications.
When Skyline is trying to figure out the actual modification that is represented by a particular mass number, Skyline will use whatever digits of precision are there.
When you create a BiblioSpec spectral library (.blib file) from your peptide search results, the .blib file does not actually know anything about the modifications other than their mass. If you try to look at that spectral library in Skyline using the "View > Spectral Library" menu item, Skyline will try to guess what the modifications are based on their masses, and users will be prompted if those modifications do not currently appear in the lists at "Settings > Peptide Settings > Modifications".
-- Nick