Your router routes. It also assigns IP addresses to devices. So no, you can’t just connect a switch to a modem and be Online. It may be possible if your ISP gave you multiple IP addresses and you set them static to your devices, but you also do not want your private devices directly on public Internet.
You can’t adapt it, but you can check the back side of the outlet. If it uses a Cat5e or better cable, and there are no daisy chained wired to it, you may be able to. Once you confirm that, you’ll of course need to find where the other end goes to as that would have to go to your router/switch.