Hello,
Sorry for the late response. You may already have found your answer -- just in case you did not, the subroutine VISCOS in 3D can do the trick.
However, I would not advise this as there is not much physics in doing so. Rather, you can use the Smagorinsky turbulence model, which set difference viscosity (for the hydro) along and across the flow.
Note that since you have the whole source code, you are free to do what ever you like ...
Hope this helps,
Sébastien.