this is actually possibly if your conductor has a thickness. In this case, you can refer to the example "capacitor.lst" that you can find in the FasterCap sample directory (see the online embedded help for FasterCap for the location of this directory). I report the list file here for your convenience, see the directory for the referenced geometry description files if you need them.
* This capacitor is made by a dielectric material with relative permittivity
* equal to 3.0, sandwiched between two square metal contacts
* with finite thickness
* lower contact
C capacitor_contact_sides.txt 1.0 0.0 0.0 0.0 +
C plate.txt 1.0 0.0 0.0 0.0 +
C plate.txt 3.0 0.0 0.0 0.2
* dielectric medium
D capacitor_diel_sides.txt 1.0 3.0 0.0 0.0 0.2 0.5 0.5 0.5 - 0xa793b4ff
* upper contact
C capacitor_contact_sides.txt 1.0 0.0 0.0 0.8 +
C plate.txt 3.0 0.0 0.0 0.8 +
C plate.txt 1.0 0.0 0.0 1.0
So you actually divide your conductor in set of panels, according to the dielectric they are in contact with; and then merge them together in a single conductor using the trailing '+'.
If instead you would like to specify a zero-thickness conductor at the interface between two different permittivity mediums, I'm sorry but this is not supported at present. Most of the time, however, for realistic models you actually have a 3D thickness, so this is not a limit.
If you believe on the other hand that this is a must have feature for you, we are open for discussing a possible extension to FasterCap to support this boundary condition as well. In this case kindly contact me again via email, and I'll be happy to discuss the details with you.