Taken from the manual:
Your case is the opposite to the example below.
Changing video frame rates
You can use the speed control to change the frames per second (fps), or framerate, of a video. If you are rendering your video to a sequence set, you can effectively increase or decrease the number of individual image files created, by using a Global Speed value less than or greater than one, respectively. For example, if you captured a five-minute video at 30 fps and wanted to transfer that to film, which runs at 24 fps, you would enter a Global Speed of 30/24, or 1.25 (and Enable Frame Blending to give that film blur feel). Instead of producing 56030=9000 frames, Blender would produce 9000/1.25=7200=56024 frames. In this case, you set a Sta:1 and End:7200, set your Format output to Jpeg, 30fps, and image files 0001.jpg through 7200.jpg would be rendered out, but those images ‘cover’ the entire 9000 frames. The image file 7200.jpg is the same a frame 9000. When you read those images back into your film .blend at 24 fps, the strip will last exactly 5 minutes.