Yup. As far as I know, you can just go ahead and use the disks. You'll have to reformat them to 800K, of course. The way disk drives differentiate between an 800K disk and a high density (1.4MB) disk is by looking for a hole in the upper left corner (opposite the write protection hole). The kind of drives that do this appeared in the late 80s early 90s, as I recall, so earlier Macintosh models would only see an 800K disk. But if you want to transfer files from the Mac Plus to a newer Mac, you'll need to cover up the hole with a piece of tape or something so it is seen as an 800K disk and not a high density one.
Also, I just found an article at http://shiva.soltec.net/~cbsc/512k/14mb.html
that describes just what you want to do.