You CAN convert existing MJ to EU that way. Just direct your Conductive Pipes/Conduits to a Magma Crucible + Igneous Extruder combo and have it output to a Geothermal Generator. Again, it goes in bursts, but if that's not using your MJ network to produce EU, I don't know what is. It's definitely more direct than Biogas since all it takes are setting up the machines and then just routing BC power into the crucible.