The M&P 9 Pro is set up for IDPA/SSP and USPSA Production competition.
It is not available with an external safety, and I can't think of a more useless modification for this specialty competition model.
Other M&P models are available with external safeties, mostly to satisfy department requirements, not because the design requires them.
http://www.smith-wesson.com/we...r1=EQUAL&isFirearm=Y