I installed the Winrar trial, and I don't see that it's possible with the product. You can select files and folders and choose "Add to archive..." from the Winrar context menu, and the only choices it offers are RAR, RAR5 and ZIP. I don't see that it supports .xpi out of the box at all.
In my ignorance I looked up what xpi files are, and they appear to be plain zip files, preferably without compression, that include a install.rdf file, that are used to package Firefox extensions. Do you create extensions a lot? Or is there some advantage for naming archives with .xpi?
It looks like send-to sends a blank-separated list of files to a script. Create a batch file in C:\Users\whoever\AppData\Roaming\Microsoft\Windows\SendTo named tester.bat and add these two lines:
and then select several files and send-to the tester.bat entry. You'll see the list contains the full filenames separated with a blank, with surrounding quotes if needed. I'd have to play with it a bit but should be able to write a batch to add files to a zip and then rename it to .xpi. I don't know that I'd use for, and I'd probably opt for the info-zip zip.exe for simplicity although I'd look at winrar's command line options.