Sorry, but I don’t get the problem. The only reason to define DIFFREALGDPSS using the pound operator # is if it is a function of deep parameters. But then you can use the RHS of the expression in the steady_state-section.
An alternative is to move everything to a steady state file. Define the expressions are parameters and just update them in the steady state file. See [Why is _steadystate.m important when estimating a model) for a starting point.