No, there is no adapter available for the push-in connector of Bullet/ splitshot/ Tripleshot to 7- or 9-pin screw-in connectors. Luckily I might add. I can see why one would put such low-quality transducers like Bullet, Splitshot and Triplshot on entry-level units, in order to keep the cost down. But who would want to put those less-than stellar transducers on good units like Elite and HDS? It would be like buying a brand new bassboat and then a padle to power it...
If your transducer in the bilge is indeed a Splitshot, you have a lot to gain from not using that transducer anymore. It is a low-quality transducer when transom-mounted, and as a shoot-through it is probably close to beeing useless. Your downscan will at best have massive distortions from the fiberglass, and your 2D will suffer from signal-loss made worse with the huge coneangle and low power of Hook2/ Splitshot. Mounting a Splitshot as shoot-through shows a lack of understanding of how sonar works, I hope this was not done by a dealer.
My boat is a brand new 2020 RANGER 1850MS and the transducer was installed at the factory. I have to admit that I was shocked when I saw the transducer expoxied into the bilge. It looks like another cost cutting measure by Johnny Morris and his White River group to line their own pockets. Crappy electronics = MORE PROFIT.
One tends to get what one pays for. :) But I would demand a new transducer to replace the one they have misplaced, as that is clearly their own mistake and it should never have been used as a shoot-through.