Like adding to entrances of Windows xp registry in Windows 98?

You can convert Regedit5 files to Regedit4 files with Regeditr for example but if the settings contained in the Regedit5 file are XP specific, they won't have any effect on 98/ME once converted to regedit4 apart from adding stuff to the registry as Windows will not be able to make sense out of them.

REGEDIT5 is simply a file format that supports Unicode entries. While you can't use actual regedit5 files in regedit4, you can none the less save the file as regedit4, and import it into Windows 9x.

Be aware that some settings that affect the implementation of win32 do not carry across well. This includes windows driver settings, vxds (9x), services (nt). The registries are none the less the same structure, even though the actual hive-files are entirely different names and locations.