Encrypt External Volumes With Pre-Existing Data

I have 2 external HFS+/GPT 10TB drives (1 thunderbolt and 1 usb) with very important data that I would like to encrypt. In High Sierra, how may I safely do so without getting the drives converted to APFS? Are there any terminal commands that I may run or is it just not possible using Apple's native tools? Obviously, copying 20Tb of external storage data to yet another location and then formatting them with encryption and copying it back is not an acceptable solution.