When image sequence is used as source, you can edit the frame rate to be your desired number. I needed to set mine to 12fps, but it took me a while to figure out you can click the dropdown, which turns it into a text box where you type into the box your desired amount.
My suggestion is to add a dropdown menu item in the list called ‘custom’ which prompts the user to type into the search box when clicked. That kind of option was what I was looking for. Having it there might make the user experience more clear.