Before arriving at my initial duty station, can I request joining a unit that deploys often?

You can try when you get to Reception, but the chances are not likely. Your orders will tell you where you are going. Now, it could very well be that your new unit could be ramping up to hit the ramp. But, even if they don't, stay in long enough and you will have your turn.

Just focus on reporting to your first duty station, meeting your new chain of command and focus on your new unit's mission. Don't worry too much about deployment because if you do get a opportunity, be prepared to train up for it.

Be careful for what you ask for, it may come true. Pentagon planners have unit deployments scheduled years in advanced, pending any urgent events. Costs lots of money to deploy. Training cycles are affected. Resources have to be in the right place at the right time. Be patient. Enjoy your time at Fort Knox.