Convert hive file into .reg file?

Many file associations are lost. Need to replace the entire subkey
HKLM\Software\Classes with another previously saved subkey. The replacement
was saved via Save Key from a backed up hive that was previously loaded and
unloaded in regedt32. Although I'm Admin, regedt32 is not giving me
privilege to do Registry > Restore on the Classes subkey with the previously
saved replacement.

Note that Load Hive is unavailable on selected subkeys; you can do it only
at the root of the HKLM tree.

So my workaround idea is to try to convert the extensionless subkey file
(Classes) into a .reg file and then import it. Is there a way to convert a
registry file into registry script file (.reg)? Note that I tried simply
putting a .reg extension on the Classes file, but when I double-click on it
a popup error states that it's not registry script. I've also tried opening
the extensionless Classes file with Notepad via Open With, but the Notepad
window doesn't show registry script.