Whilst behind the scenes when a disposal schedule is created it is treated as a separate object, there is nothing in the user interface which allows you to select a previously defined disposal schedule to attach to a new category.
Having said that while you are setting up a fileplan you can use the copy/paste functionality on a category which will copy all of the information, including the disposal schedule. You can then make any adjustments to the category just copied. Obviously doing this in a system when you have folders and records would also copy those as well.