Fishing boat alternator problem

I have a 24v alternator on a small fishing boat charging either of two pairs of 12v batteries independently selectable by switch. 12v electrics are powered via a 24v/12v convertor. I am now required by law to have a totally independent battery exclusively for the 12v radio.

My problem is charging this battery. I don't think I want to install a second alternator. Is there a satisfactory way of charging a 12v battery with a 24v alternator? Could it be charged via a 24v/12v convertor? Or do I need to install another pair of 12v batteries and connect them to the radio via the convertor? If so can the new batteries (presumably smaller) be safely charged in parallel with the main batteries (one pair of main batteries has to be on stream the whole time to run the rest of the boat's electrics)? Advice on the best way to tackle this problem would be gratefully received.