You would need to install a pump, usually a macerator type. There are a number of ways to plumb it. I installed a tee in bilge in the pipe that goes to the deck pumpout. Then to a macerator pump, then to a y-valve on the discharge seacock in the head, to select between emptying the tank or direct discharge from the head.
I did something similar. I have a LectraSan behind the toilet in the head. I can discharge overboard there or send it to the tank. I put a large seacock in the wet locker forward of the chart table and a Y valve and a hand pump (feared an electric pump would fail on me). In can pump out on deck or discharge through the seacock.
I can change it to go directly out instead of going in the tank but once it’s in the tank I can only pump it out, not discharge overboard. I’m trying to figure out if there’s another way to get it out of the tank once the 1&2 gets in there.
I had it a manual pump similar to the Morgan supplied bilge pump. It is hooked up to a Y valve in The area under the head sink. Additional plumbing runs to a whale pump installed in the cockpit on the starboard side opposite the Morgan bilge pump. The discharge hose runs through a hole above the waterline on the starboard stern side.