They were originally installed with nut-serts. Back in the day, you could get the roof rack as a dealer option if it didn't come with one. You would lay the rack on the roof and use a marker to mark the holes. Then you would remove the rack and center-punch and drill the holes, being careful not to drill through the headliner. You would file the holes a little and coat with touch up paint to prevent rusting. You would then tap the nut-serts into the holes, install the rack, and use a special tool that would collapse the nut-sert (somewhat like a pop rivet) and you tightened the bolt. The nut-serts and tool came as part of the roof rack kit. I'm not sure if the roof rack kit is still available from Nissan (I wouldn't hold my breath), but you could try locating the part number and checking with Nissan to see if they have one somewhere. You could also check for aftermarket options. Try
www.4x4parts.com