Currently you have two options:
1) Create a chain of Merge Nodes that feed a F0 Through in parallel to the Material Regions
2) If you have an unmapped/unused slot (for example you are using bump bot not normal), use that slot.
Technically it isn't a problem to add another slot to the SG Regions.
One of the Float values of the Outputs (A,B,C,D) is still unused so it wouldn't add more Outputs.
However unfortunately Material Regions are not greatly optimized when viewed in Shaders since for each Shader Slot (Let's say you are viewing a shader with Diffuse, Gloss and Bump) you are always evaluating the full material region.
Meaning that for Diffuse you are calculating EVERYTHING inside the Material Region not just the diffuse. So bby adding a F0 Specular your expense (and risk for red screens) goes up by factor 6 or 7.
How many Material regions can you currently add before you get a red screen ? Out of curiosity.
Message edited by Campi3d on 1/18/2017 - 2:56 PM