How do I encrypt an external HFS+ drive without erasing it on Big Sur?
Apple says you need to remove the disk from being a Time Machine destination for your Mac first.

If you want to change from unencrypted to encrypted backups, you must remove your backup disk and then set it up again.

https://support.apple.com/en-gb/guide/mac-help/mh21241/mac
Removing the disk doesn’t delete the contents of the disk though, so you should be able reassociate the disk with your Mac once it’s encrypted, though I haven’t tested this.