Unfortunately I think currently there is no way to customize the FPS on Shotgun Create and it only read the original frame rate of your video, though it is supported in RV.
Sorry for welcoming you with an unpleasant answer. You could follow the post here to submit it as a feature request directly to our product team.
If the image sequence files has no frame rate embedded in its metadata (ex. in jpg), Shotgun-Create will allow you to set it by right-clicking on the frame rate indicator on the right side of the timeline widget.