I don't think you should be trying to create .ssl files.
It is possible to convert plink crosslinked peptide search results to proxl, so I think you should do that.
Here is the project page for the tool which converts plink to proxl:
In the "How to run" section it tells you where to download the file "plink2toProxlXML.jar" and how to use Java to run the jar with commandline arguments to convert a .plink file to proxl.xml
Once you have proxl.xml files you can use those to build a BiblioSpec spectral library.
In Skyline, you can use the "File > Import > Peptide Search" menu item to go through the process of creating a spectral library from proxl search results and then importing proteins from a FASTA file and extracting chromatograms.
Let us know if you get stuck trying to convert your plink search results to proxl.xml.