Is there a paddle that converts from a SUP paddle to a kayak?
Scott Burke makes one that goes with their cross-over Ky/Pad board.You just push the little nipple, remove the sup paddle handle, and slide on the blade, which clicks in place.
Neither look very good for serious paddling. Lots of gimmicks are passed on beginner paddlers. Best SUP paddle is strong, light, fairly stiff, and hydrodynamic in the way it pulls your craft through the water without flutter or turbulent flow. Avoid plastic and aluminum paddles. Good entry level paddles are usually fiberglass. If you paddle several times a month a good carbon paddle is worth the investment and actually a bit more important than the SUP board you use.
I prefer carbon paddles for SUP paddling, but I recently bought an inexpensive paddle that adjusts to a small size so my sons and their wives can paddle my old SUP board when they visit. It’s a Werner vibe and it has an injected molded blade and fiberglass shaft. I’ve surfed with it a couple of mornings and taken it for some long paddles. It’s very stiff for a plastic paddle and light. It’s the best cheap paddle I have tried for SUP and at $110 pretty affordable for family paddlers.