Yes, you can walk between terminals at Hartsfield-Jackson Atlanta. Keep in mind the terminals are a half mile in length. I'd recommend riding the Plane Train.

In Hartsfield-Jackson Atlanta International Airport (ATL), you generally cannot walk directly between the Domestic Terminal (West) and the International Terminal (East) if you are "landside" (before security). The two terminals are located on opposite sides of the airport's massive airfield, separated by miles of runways. If you arrive at the Domestic Terminal but need to check in for an international flight, you must take the complimentary terminal-to-terminal shuttle bus, which takes about 15–20 minutes. However, if you are already "airside" (past security), you can easily move between all concourses (T, A, B, C, D, E, and F) using the Plane Train or the moving walkways. Concourse F is the international terminal; while you could walk the entire length from the Domestic side (Concourse T) to the International side (Concourse F), it is a high-value distance of over 1.5 miles. For most travelers, the Plane Train is the peer-recommended "fast track" to ensure you don't miss your boarding time.