You add a custom process step and enter the command to run and its arguments.

If you want to share these steps with your coworkers then you might be better of to integrate them into your build system though. Custom process steps are stored in the .user file and that is not meant to be shared.

Take a look at the qmake documentation [doc.trolltech.com] and the Undocumented qmake [qtcentre.org] page on QtCentre. (This functionality has always been documented, but IMHO the QtCentre page is clearer. (Disclaimer: I wrote that wiki page…)