I don't know of a way to embed an external file into an excel file.
My first thought would be to copy the text in the XSLT file to one of the sheets in the Addin or embed it into the VBA code. I'm guessing that there are macros in the Addin. The first test before any code is run that needs the XSLT file is to see if the file exists. If it doesn't, then write the file on the fly. Style sheets are relatively small, right? It could be written to the addin folder or maybe a folder selected by the user.
Jeff