Couldn't you find the answer in this article?
The 2D panel is not modelled in the default C172, so you would have to do everything yourself.
For the 3D panel, I think it's a very long process to edit that and I'm not 100% sure, but it might require some coding.
In the meanwhile, a good work around would be to use NAV1 DME on the GNS530,
And the NAV2 DME could be simulated with a "Direct to", the D with the arrow, on the GNS430. You would get a GPS DME.