FastFileSystem won't be found in L: on OS 3.1, because it's located in the kickstart ROM. (Though updated versions can be found there in later OS releases.) CrossDosFilesystem is just an included filesystem that allows you to mount and read/write from PC formatted floppies.
You could probably use the RomSplit tool from
Doobreynet to grab your ROM, separate the FastFileSystem module, apply the multiuser patch, then stick it in your L: drawer.
PFS3 also includes a multiuser version, though I've never tried it.