How to encrypt external drive without erasing data in macOS?

I'm running Filevault on my internal hard drive, but I realized that I never encrypted my external drive and I'm not seeing any encrypt options anywhere (right click in Finder, disk utility, etc...). How can I encrypt this drive in place without having to erase all the data first?

In this situation, Bitlocker is the best choice. Although Microsoft has not implemented Bitlocker client for macOS, but with the help of M3 Bitlocker Loader for Mac, we can still use Bitlocker to password protect/encrypt external drive in macOS Mojave/High Sierra/Sierra and Mac OS X.

Tutorial to password protect/encrypt external drive with Bitlocker in macOS

Step 1: Download and install M3 Bitlocker Loader for Mac.

Step 2: Insert an external drive (Such as USB drive, flash drive, memory stick, external hard drive) into Mac computer.

Step 3: Launch M3 Bitlocker Loader for Mac.

Step 4: Click "Encrypt" button in toolbar of M3 Bitlocker Loader for Mac.

M3 Bitlocker Loader for Mac

Step 5: Select external drive and then click Encrypt.

Encrypt USB drive with M3 Bitlocker Loader for Mac

Step 6: Enter the password.

Step 7: Save the recovery key.

Step 8: After the encryption completes, the external drive will be encrypted/password protected.

After encrypting external drive with Bitlocker on Mac, we can use M3 Bitlocker Loader for Mac to read/write Bitlocker encrypted drive in macOS.