Level 6
By skillfully reworking your powered armor’s systems and making minor modifications to the chassis, you can fit more improvements onto the suit than its design ordinarily allows. You can add either one more armor upgrade to your powered armor than its normal maximum of upgrade slots, or one more weapon than its normal maximum number of weapon slots. At 12th level, you can have both one extra armor upgrade and one extra weapon simultaneously. This feature stacks with similar abilities, such as the enhanced tank technique of the armor storm soldier fighting style.
Source
Character Operations Manual pg. 106