In the north they did not have slaves they were free men. Once the 54th in the south I don't think they just came up to sign with them. How would they could try to escape at night and hope they got away other wise they would be sent back to where they came from. They would need to be trained you did not do that in the field you would be trained then sent out to fight. They would need to understand what they would need to do make sure they could march how to load the gun with out killing themselves by putting to much in the gun or not enough. Then to follow orders knowing that they would be going back into the south and if captured they would go back to where they ran away from or killed for fighting for the north.