Dave, no in order to open a .fp5 filemaker file FMP11 will have to convert it to .fp7 format. Those use FMP 5 or 6 will not be able to open the .fp7 files.
You may want to look into the volume licensing and bring the whole company up. We just made the same jump several months ago. It's never easy to get everyone to change software at once (or cheap), but I think the improvements between 5.5 and 11 are well worth it.