Sure it's doable, but it's not very efficient. Many folks (including me) do this all the time when we plug in our laptop
chargers into a small inverter and plug that into a cig socket. You're going from DC-AC back to DC... just because the plugs all fit and the voltages and charge rates get controlled. And in the process lose a shitload of efficiency.
Since you're going from one 12vdc battery to another 12vdc battery, then just use a set of clips like the poster above suggested, or (like I've done - i also have a trolling motor) wire in a permanent, fused 12 V power line with connectors that you can plug in to your dinghy
battery when needed, but will stay coiled up under cockpit
seat or locket lid when not needed. No conversion issues or losses.
Note that you will still be draining house batts, but if you're running the main engine
to charge then you should be fine (assuming the alternator
is up to the task of adding another battery and you aren't coking up your engine by running under too light a load for too long.. yada yada).