Im going to say this and then everyone will poke holes in it. Plus im not going to do the math to see if it will balance out. Lets say Kethane is a analog for Ammonium Nitrite NH4NO2, how ever the converters break it down, contains the component need to make Hydrazine (N2H4) as a Monoprop , LH2 for fuel an LO2 for an oxidizer. Kerbal might have the same respiratory setup humans have, inhaling Oxygen exhaling CO2. The Soil the plant is in might contain Nitrogen and have a water recycler to water the plant and recycle the transpiration from the plant. The exhaled carbon can be used to create simple sugars along with the Hydrogen in the water to sustain the plant and instead release Kethane, in this case Ammonium Nitrite, as a byproduct. Now this is still all complete non-sense but someone had to support it.