It depends on where the existing through hull is..
...and where the tank is. If it's close (within a few feet), you can tee or wye into the head's discharge thru-hull. Do NOT connect the macerator directly to the tank...in fact, never connect anything "hard" directly to anything else "hard ...that's a good way to end up with a cracked tank. Always "soft couple" with at least a foot of hose to act as a shock absorber. Install a y-valve in the pumpout line...the hose coming out one side goes to the deck fitting (as it does now), the hose coming out other side goes the macerator, then up and over a vented loop that's at least a foot above the waterline at any angle of heel...and to the thru-hull. The macerator should be close to the tank and at or below the same level as the discharge fitting on the tank, so it will prime quickly...running a macerator dry burns out the impeller in it. Macerators are 1.5" in (same as your tank discharge fitting), but only 1" out. Your existing head discharge thru-hull is 1.5"...so if you go back to that through-hull, you'll need either a 1" x 1.5" reducing adapter, or a 1" x 1.5" tee or wye to connect 'em. If there's already a vented loop AFTER the wye valve in the head discharge, tee or wye into that line ahead of it--between the y-valve and the loop--and you won't need another one.
