The Everyday Messenger's main strap and stabilizer strap were also designed to function as luggage straps. Depending on the size of your roller bag, one or both of these straps will work for you.
Using the Stabilizer Strap as a Luggage Strap
Option 1: Straight Configuration
How to do it: Take both ends of the stabilizer strap out of their stow pockets and hook them together. Pull on the loose end of the webbing to cinch the strap tight against the back of the bag. Then place your bag on your luggage, passing the luggage handle through the stabilizer strap. This option works best when your bag is packed with a bottom-heavy load and your rolling luggage has a flat top.
Option 2: Top Handle Pass-Through Configuration
How to do it: Take both ends of the stabilizer strap out of their stow pockets. Set your bag atop your luggage, with the back of your bag resting against the luggage handle. Take the hook end of the strap and adjust it to maximum length. Lead it around one post of your luggage handle, up through the bag's top handle, back down and around the other post of your luggage handle, and connect it with the other end of the stabilizer strap. Cinch the stabilizer strap tight. This configuration is the most secure, and best for times when you'll be carrying your luggage long distances.
Using the Main Strap as a Luggage Strap
How to do it: Tighten the main messenger bag strap as tight as it goes (see this article for strap adjustment tips). Then set your messenger on the front of your rolling luggage, with the main strap behind the base of the luggage handle. This configuration is best for large roller bags that have enough weight in them to counterbalance your messenger bag.