Reading the documentation for index rollups is see that:
You can use index rollup in three ways:
3. Specify the index rollup job as an ISM action for complete index management. This allows you to roll up an index after a certain event such as a rollover, index age reaching a certain point, index becoming read-only, and so on. You can also have rollover and index rollup jobs running in sequence, where the rollover first moves the current index to a warm node and then the index rollup job creates a new index with the minimized data on the hot node.
However when looking at the actions available for ISM, I see no mention of rollups at all.
Am I misunderstanding the documentation? Is it possible to perform rollups during an ISM state transition? If so, how do you specify the job as an action?