It's possible, but with all in-place conversions, there is a certain risk of data loss.
To make room for the LUKS header, unless you intend to use an external header, you must first shrink the filesystem (with resize2fs if it's ext4). For LUKS1, you should shrink it by 2MiB. For LUKS2, you can shrink it by 4M, 16M or up to 64M. If you intend to use advanced ...
While not quite exactly what I was looking for, I was able to get it running.
This was through Raspbian's rapsi-config utility.
Steps were as follows:
Run the rasps-config command with root privileges.
Choose the second option: "Network Options"
Enter SSID and Passphrase
Find and open your "wpa_supplicant.conf" file. Mine is located in /etc/...