We don't know that for sure. If the overwhelming majority were slaves then it can't be said that they had a choice, can it? No doubt some served the army voluntarily but it's likely that most were there because their owners were or their owners said so.
slaves could NOT be members of the army, navy or marines because they were not free to take the oath of enlistment. we are talking about FREE MEN who joined of their own FREE WILL. you KNOW this.
FRee dixie,sw