Players may make alternative characters that are hired into the Diamond Dog crew. However, the caveat is that they must be paid a normal working salary. On the flip side of that, the alternative character can be put on hold before being hired (for money and debt reasons). 

For example: Player Bob wants to make an alternate character to play. He runs through the creation process then alerts the Referee and the other players that his character wishes to be hired now. The GM and players agree to the arrangement. Bob doesn't mind his character being hired without roleplay, and chooses to be someone that the captain hired from a Wanted Ad.

Another occurring problem is absent players. Characters that are played by absent players will be relegated to "guarding" the ship. If an absent player states they will be gone for an extended period of time, the character will be left on wherever the land headquarters of the crew is located, effectively sitting out the current mission. The GM may write fiction pertaining to that character as to help the player reinsert themselves into play. 

For Example: Player Timmy and Sally need to sit out a game. Timmy knows he will miss only one or two sessions so he's relegated to crew duty. Sally knows she'll be gone for more than two sessions and tells the Referee that. The Referee writes that Sally's character spent most of her time at the crew HQ filing paperwork, doing the daily shopping, and keeping the HQ stocked. 

For more information on other house rules, please reference: this.