You should be able to copy over folders and sub-folders to a Mac-formatted drive, no problem.
You want to put the Lightroom catalog and cache files on a fast drive (like the internal SSD). Full-size photos can go on a Mac-formatted external hard drive.
I'm guessing (hoping) that there's some way in Lightroom to point it to the top of the copied folder tree (and have it take care of fixing up all of the references) instead of having to tell it where full-size photos moved, one by one.
But note that if the existing drive is formatted using NTFS, the Mac will be able to read it, but won't be able to write to it. Out of the box,
- Windows and macOS can both read and write FAT32 and exFAT
- macOS can read (but not write) NTFS
- WIndows cannot make heads or tails out of HFS+ and APFS at all