In hot embedding a carbon filled phenolic resin creates enough conductivity to prevent charging of the embedded sample.

For cold embedding only epoxy has so low outgassing properties that they can be used. However it is not possible to pre fill the epoxy with a conductive component. rather you have to do it during the embedding. Copper shots are a very good option as they have enough weight to make contact and create a conductive path. Alternatively you can drill through the embedding into the sample and put a copper screw in the hole to make contact to the sample.