Bill Bulat
Ansys Employee

I regret to say that I'm not sure there's a way to accomplish your objective. I'm almost 100% certain that the initial stress feature in Mechanical leverages the INISTATE command in the MAPDL solver to set the initial stress state:

And, as it's name implies, INISTATE may only be used to define the initial stress state at the beginning of an analysis (not to override stresses calculated at some later time). As you said, loads may be applied that vary both spatially and temporally. But stress isn't a load - it's a calculated result. 

If you know the loads that gave rise to these surface stresses, you could of course apply those. Or if you have a way of inferring pressure and traction loads (or displacements) that would give rise to your surface stresses, you could apply those. However, in a transient structural analysis, the inertia of the structure beneath the surface plays a role in the dynamic response - it would affect the surface stresses in a way we may not be able to control by applying surface loads alone.

It may help to know what created these surface stresses and/or how the stress data was obtained (i.e., more background).

Kind regards,