Most long arm heads are specifically engineered to operate on a frame. They don't have foot pedals and most of the controlling is done with the handles.
2 options I can think of which have both options available from the factory are the Grace Q'Nique machines (and their BlockRockIt twins) and the Juki QVP2200. When the Juki first came out, there was much excitement because if could be used in both configurations. Since then I've read a number of reports of frustration with the machine in general so do your reading first. Leah Day's sit down implementation of the Q'Nique 21 was the first I'd need that machine used as a sit down. Leah has since put hers on a frame, but it is at least do-able.
Heads from other brands, the more mainstream longarms like Innova APQS, Gammill, and HQ will likely cost quite a bit more to put in a sit down configuration, if the manufacturer even offers the conversion. If you don't ever want to quilt on a frame, look closer at their offerings already designed to be sit downs or plan to trade if you decide you want a frame.