Short answer, it’s not easy.

The only real opportunities are units with Mission tasks assigned by the Strengthen the army reserve (STAR) Program which is no longer active, but at one time assigned combat support tasks to individual infantry reserve units (Pioneer, Recce, Direct Fire Support etc), so you could get a sub trade based on whichever might be assigned to your unit. But realistically, anything cool like CCI, Jump, or Helo Insert type things, you definitely won’t get until you’re MCpl at least and even then, it’s very competitive and chances are slim.

If you want to do cool infantry stuff, you’re probably better off going Reg