Prescribed settlement in STAAD.pro is limited to nodes. The help topic 5.32.8 Support Joint Displacement Specification explains most completely.
If part of the wall settles more than other parts of the wall, then you'll have to break that up into smaller parts so that you can prescribe differential settlement to the nodes along that line.
It may be that meshing the walls into plates with more nodes is the way to go. For more on plates vs. surfaces in STAAD.pro, search this forum, here’s one post on the topic.