Are there round header pins?
There are definitely round shaped header pins - just try a web search for ‘round header pins’ and you will see many. However, as Chris Stratton points out, fluid tight seals are somewhat tricky - for example, you can’t just surround the pin with another solid material and achieve fluid isolation. Typically a connection would need either a glued (like epoxy) seal between the two materials or else (depending on the fluid pressure) some sort of compression seal (multi conductor circular connectors in military applications typically go this route).