Shimano 9sp MTB shifters are fully ratcheted, instead of having a forced-motion spring-detenting system, as on an old thumbshifter or on a Campagnolo Ergo shifter.

This means you will have the difficult choice to arrange the extra click in your shifter out beyond the smallest cog, where it will yield an occasional phantom-shift, or to situate the extra click up beyond the largest cog, "locked out" by the lo-limit screw.

Locking the shift out using the lo-limit can invite an overshift and an over-stretched cable tension, which can then prevent the shifter from shifting back to the smaller sprockets. You might have to release the cable attachment in that case to restore shifting.

The choice is yours. A gripshifter would work fine with the locked-out shift since it's a forced-detenting mechanism, but you'd need an ESP 1:1 rear derailer too.