Yep, it's possible...
Portapotties come in two types: totally portable--you have to carry the tank off the boat to empty it--and permanently installed models that are vented and plumbed to a deck pumout fitting. Connecting it to a larger holding tank only requires running the pumpout hose to the holding tank instead of the deck fitting, and installing a manual or electric pump in the line to transfer the waste. The pumpout line from the larger tank is connected to the pumpout deck fitting. Why do it this way instead of just replacing the portapotty with a marine toilet? Lower cost if you already have the portpotty and just need more holding capacity. No thru-hull and seacock for flush water intake--the portapotty has its own flush water reservoir that can be refilled from the sink with a pitch if necessary. No seawater to get trapped in the head intake and stink. And, the extra holding capacity of the portapotty tank plus the larger tank. None of the maintenance required by a marine toilet--no moving parts, nothing to wear.
