There is no direct way to model earth pressures in RAM SS. Lateral loads of the type "Other" are limited to User Defined Nodal Loads, or User Defined Story forces (i.e. diaphragm forces).
If your concern is the design of the frames to take this extra force, then you can enter the total lateral force as a User Defined Story Force in RAM Frame. This works with Rigid or Semi-rigid diaphragms. Note, the positive X direction towards plan right is 0 degrees and angles are measured counter-clockwise from there. You will have to use custom combinations to have these "Other" type loads combined with gravity, wind and seismic effects. Take care not to use negative load factors on other type cases, assuming the lateral pressures are in one direction only.
If you are more concerned with effects on a single frame, then nodal loads (with a flexible diaphragm) might be best. These are applied in the Modeler - Elevation View to the nodes of the lateral members.
What the program cannot do at this time is design a foundation wall for the earth pressures applied to it out-of-plane. Ram Elements and the retaining wall module might help there.
RAM SS - Rigid Diaphragm Constraints and Frame Shear [TN]