I assume you've already tried big-big combo since you're saying it's "short"? SRAM advises +2 for different scenarios people do not take into account (I.E. - chain stay length changing on full suspension bikes).

There's no easy way to add more links other than using a second PowerLink. If you've already tried big-big, and you can still push the lower derailleur cage up some (it's not totally maxed or under tension), I would just leave it as-is rather than add links (because you should not be riding big-big anyway), and it shouldn't cause damaged if you accidentally shift big-big.