How to take an exposure longer than 30 seconds on a Nikon D7000?

I can put it on bulb mode, but I have to hold the shutter button down. There has to be an easier way

You should have a manual setting for your shutter speed that you can manually start and stop. But make sure you use a wireless of remote button otherwise you are going to get some blur due to the camera moving if you push the button on the camera.

Nope, that is how bulb works.

Get a remote, such as the ML-L3 from Nikon. They cost about $15. Set the camera to manual mode, set the aperture, focus, and ISO. Then Set release mode to remote (or timed remote) and then to what 'bulb' would have been. This setting is now called "Time". Press the remote button once to open the shutter, and once to close it. With the remote, you can take long-exposure photos with shutter speeds up to 30 minutes.

Make sure that long-shutter speed noise reduction is turned on, as it will prevent the purple edge fringing and hot pixels that can occur. Unfortunately, it will take another 30 minutes to process.