We have a variety of overlay materials but one of the most numerous is our road patches, which have been designed to work in tandem with our city street materials so we'll be using that as our example for this tutorial.
The same principles would apply to almost any overlay that was designed to be used as a complete material. (I.e. One that includes a full set of PBR maps.)
How to apply an overlay
We're going to assume that the road material is already imported via the converter and mapped how you like it.
Via the vertex panel, create a new UV map and name it something easy to remember, in this case, I used 'Patch'. This will be a separate UV just for our overlay.
With NO object selected, load in the overlay material using the material converter. As there is nothing selected, it will nto be applied to anything but will be loaded into memory.
Return to your original material and press Shift+A inside the node editor, navigate to the groups menu and find the patch material you just imported. This will create a nodeGroup similar to your original material.
Replace the original Coordinate node with a UV Map node and set it to 'UV Map', this is the default UV mapping for the original material.
Add a second UV map and connect it to the vector input of the overlay material we just added, set this one to 'Patch', or whatever you named the new UV Map.
Press Shift+A again and add in a PBR Mixer node from the Group menu.
Connect up the first set of Color, Roughness, Normal and Displacement outputs from the original materials into the first set of inputs of the PBR Mixer then do the same for the overlay material into the following set of inputs.
Connect the outputs of the PBR Mixer to your shader and displacement nodes (See above.)
Finally, use the alpha output of the overlay material as the factor value for the PBR Mixer. This alpha output will tell the mixer when to use the original material and when to use the overlay.
All that's left now is to adjust the patch UV to place the overlay and adjust any displacement settings on both materials.