Create a new Impose node and edit the Imposition Preset so that under the Press Sheet tab, the Press Sheet Size is set to Automatic. Doing this makes sure that you can use any sheet size in this node, so you can reuse the node in other workflows or job types.
Under the Layout tab, choose Page Order to be Manual. Then in the table set the Page to be -1 and the Increment to be 1. You don't have to add any other lines as using this it will place on the first page the page -1 (which is the last page) and then on the next page it is going to increment this by one in this case as the original value is negative it is going to increment backwards, so the next page will be -2 and so on.
Other options you can leave as defaults (i.e. 1 Row, 1 Column, no scaling, no clustering etc).
Now when you submit a job, this node will reverse all the pages in the document.