Is it possible for a reservist to deploy with a national guard unit? How hard is it to get done?

If the deploying unit has an unfilled requirement and a request for a fill, then you might be lucky if, and only if, you apply they the appropriate channels (whatever system is in place to volunteer as a reservist) you get picked.

1. If they request a fill through MOBCOP Tour of Duty that is open to USAR and you apply and are selected.

2. If they don't have enough fills of an MOS through the guard you could be assigned to fill their unit at the Mobilization site.

3. If you want to go with a unit it's not hard to change component.