Not until your current enlistment expires. After that, you can reenlist into the National Guard and apply for an AGR position, although I can tell you they're not particularly easy to get.
Have the regular Army recruiter help you fill out a DD368. Give the DD368 to your chain of command to sign. Bear in mind your Natioanl Guard does not have to sign the DD368 if the don't want to....
No, you must terminate your active duty statutory obligation (the period of active duty that you had signed your contract for) and then you have the option to enlist into one of the reserve...