RBEs correspond to coupling constraints in CalculiX (RBE 2 = kinematic coupling, RBE3 = distributing coupling). They were requested in the past, e.g., here: RBE3 integration in PrepoMax (Kinematic & Distributing Coupling)
For now, you could use a rigid body constraint to apply loads/BCs to a face. This would be equivalent to RBE2.
Coupling constraints currently require keyword edits. But you have to be aware of their significant limitations in CalculiX: Different coupling constraints and their limitations - CalculiX (official versions are on www.calculix.de, the official GitHub repository is at https://github.com/Dhondtguido/CalculiX).