MacOS – Can Time Machine volumes be easily encrypted with FileVault 2

No, the backups will not be encrypted automatically, but it's very easy to enable for directly attached disks. Just check "Encrypt Backup Disk" in the Time Machine disk selection settings.

If you're backing up to another Mac, you can use Disk Utility on that Mac to erase non-boot drives and put an encrypted partition on them:

Ideally, the CoreStorage encryption should continue after you reconnect the Time Machine drive after your format. But practically, it's a risk that you shouldn't take unless you have other backups to recover from (like a SuperDuper! or Carbon Copy Cloner bootable clone).