I don't know if it's a good idea. A diode isolator and transfer switch might be okay -- but the question is chemistry, terminal voltage, and charging current specifications.
An automotive alternator can produce very high charging currents which some batteries cannot tolerate. You need to look at the chemistry of the battery (is it lead-acid?), the amp hours, and hopefully some kind of specifications.
With that, Luke, we can give you an intelligent answer...okay we can give you a THOUGHTFUL answer.