The following CLI command works:

/host=myhost/server-config=myserver:add(group=group-name,socket-binding-port-offset=2000)

but the offset of 2000 is ignored (does not appear in host.xml). I realize it doesn't make sense to define an offset without a socket-binding-group defined as well. I think the CLI command should have failed and displayed a message like "socket-bind-port-offset requires a socket-binding-group to be defined" or something like that.