If a modded furnace doesn't extend the vanilla furnaces, the mod (or a compatibility add-on) needs to provide instructions for the Exoflame to operate that furnace-like block. To do that, it needs to implement the ExoflameHeatable interface, i.e. this needs Java code.
Implementation should be reasonably simple, considering the Exoflame only needs to know four things:
- a test, whether the furnace can currently smelt anything
- information about how long the furnace will continue burning (with or without actually doing anything)
- a method to increase the remaining burn time, which will be called if the furnace can smelt things and is about to run out of burn time
- a method that will be called every other tick that will advance the smelting process of any items in the furnace
Botania does that for the vanilla furnace block entities, which might be a good example for compat add-ons.