This plug-in may work for what you're doing, based solely on the screenshot that shows a list of extensions to import, as I have not used it: robcole.com - OttomanicImporter
The author posts on here quite a bit as well, so you can PM him and ask any further questions, offline.
If this is in a single folder or only a handful, then I'd probably just move the non-JPGs somewhere outside of the folder, temporarily, so LR cannot see them, do the import, then move the non-JPGs back. There might be some way to make the images invisible to LR using security, as well, though I'm not a Mac person so don't; have any specific advice.