Time stretching

You have a couple of options. The first option is to drop your footage in a 30-minute comp and go to the Layer/Time/Enable Time Remapping menu or use the keyboard shortcut Alt/Option + Ctrl/Cmnd + t. This will add Time Remapping to the timeline and set a keyframe at the first frame of the footage and another one after the last frame, and it should extend the footage layer to the end of the 30-minute composition. All you have to do is drag the second keyframe to the end of the composition + 1 frame. 

The second option is to do some math and figure out a new interpolated frame rate for your composition. One hundred frames over thirty seconds is 0.3 frames per second. Select the footage in the Project panel and then use the File/Interpret Footage/Main menu or select the footage and use the keyboard shortcut Alt/Option + Ctrl/Cmnd + g. All you have to do then is set the frame rate to 0.3 fps, and the footage will now last thirty seconds.