Swen has said this is not possible, some time ago... IIRC for the original D:OS. With the increased emphasis on relationships between the characters in D:OS 2, swapping characters in and out of the game is even less feasible.
Relationships between NPCs and party members are tracked, and dialogue / quest choices and character development can influence each other. Having one new character drop in and make arbitrary choices and actions results in non-trivial consequences for the development of the character that was replaced (what happens they were not present for a key part of their personal quest, etc?). That doesn't even consider the issue of scaling characters by level (it isn't just equipment; there are also skills, tags and attribute/ability/talent point distributions to consider) or exploits of bringing unlimited gold and potions, etc, into a game.
The release date has been set and the focus now is on finishing the game, not adding new features, especially ones as significant as this would be (for example, no game systems are written to account for the possibility of multiple characters with the same unique origin story).
