Many of us would would also appreciate an answer to that question.
When the Form 3 launched, there was lots of signaling from Formlabs that Open Mode would be supported and that they were simply working on other, more pressing issues first. But some time later, references to the feature seem to have dried up and some users have expressed skepticism it will ever surface (they suspect the closed resin ecosystem is a cash cow for the company).
I’m more optimistic myself, but would really appreciate an official response from Formlabs affirming their intentions (and maybe a ballpark timeline, e.g. early/late 2021, 2022, etc). It would be nice if they could state this for their whole lineup (3, 3L, 3BL).
In the meantime if your resin profile is close enough to an existing one, you might be able to use an empty cartridge or the Universal Cartridge (google it) to “trick” the printer (not officially sanctioned, of course).