It cannot be modded to allow it to happen. That will leave you with 1 game (the host's) with a full custom party. Once done and in game, just shut down the extra copies of the game. You can create extra custom characters at the start by running multiple copies of the game on 1 computer and joining together in a LAN game at character creation to create the extra characters.
Or after loading the save they can invite someone else, or 2 or 3 other people, and assign control of the party members to them. The host can load up that save and play it solo, controlling all characters if they want. The host is the only one who own's the save and all characters are locked into that save. The control assignment can be changed anytime. As they join, the host can assign control to either themself (the default) or anyone else who joined the host's game. Beyond that you need to find npcs to ask to join your group. When you both finish creation and enter the game you have a party of 2.
They will also get to create 1 character. The host sends an invite and the other player joins while you are still in character creation. At that point the host can only create 1 character. Host starts the game and gets to character creation.
