Rather than re-creating macros from scratch, I'd like to copy macros from a previously used .mac file to a current .mac file. Is there any way to do this?

I'm using version 10.20c

It's been a while since I used that version, but I believe there's a command in the Macro menu for "Load and append to existing...". It appends the contents of another macro file to the one currently loaded. Make sure you save your macro file after doing this: something I learned the hard way.