It doesn't work that way. SRB2 is programmed to open the PLRs there, and only those. You COULD make them "PLR files", but that's pointless, really. PLRs are just wad files with a renamed extension. You'd have to reprogram SRB2 to load which ones you want. And, no, you can't edit the PLR files. At least, not without breaking SRB2.