"Add build step" under the "Configure" settings in Jenkins...

This may be used to add another build step such as calling a NAnt build. Two such steps are shown in the image below. The "Execute shell" and "Invoke Ant" steps below may be drug on either side of each other by the sets of 16 grey dots at their titles to reorder the build order. The "Execute shell" step may be run to use a GRUNT script with a "Command" setting such as this:

cd build/foo
npm install