Possible, yes but likely very tricky. The UI would need changes (party formation buttons, character button sizing, etc) which should be straightforward but more complex issues would be game balancing (scaling up enemies to challenge a 6-size party) and pathfinding (the more characters you have, the greater the likelihood of them getting in each others' way).
Given how many problems the Infinity Engine series (Baldur's Gate, Icewind Dale) had with pathfinding, I suspect this would be the biggest issue.
