Filevault and External drive

FileVault is not really the encryption part. You can encrypt a drive using the contextual menu in Finder or using Disk Utility.

The FileVault part is really the ability to decrypt the drive and login using the same credentials. The encryption part is the same encryption used by Disk Utility.

If you encrypt from the Finder, it will likely convert the drive to APFS.

I think Disk Utility will require you to erase the drive and format it encrypted.

You might be able to use the diskutil command in terminal to encrypt the drive preserving the contents and format, but I'm not certain. I'm not capable of checking all the options at the moment to give you definitive answers. Looking at the man page for diskutil, it doesn't appear that encryption of anything but APFS is possible anymore. The CoreStorage hooks may still be there but not documented, but I don't know.