Encrypt external HDD without erasing data on High Sierra?
Normally the option to encrypt the drive can be found by right clicking the drive's icon in finder and selecting 'Encrypt "DISKNAME"'. You will not see the encryption option if the disk is using Master Boot Record or Apple Partition Map as the partition scheme. 

This is because High Sierra encryption actually converts the Mac OS Extended (Journaled) to Apple File System (APFS), and this cannot be done on a Master Boot Record or Apple Partition Map partitioned drive.

You can check the disk partition type in Disk Utility.