If you're not using the migration tool then there is no other official tool to perform a migration. It means you can build an ISE deployment at your leisure, and then configure it up until you're ready to point all of your NAS's to ISE. Perhaps start with one device of each device category to test your policy sets.
You can export the ACS network device list into a CSV, and then import that into ISE. A bit of Excel copy and paste required. But that's quite possible. But you won't be able to export much else. You'll end up building ISE Policy Sets from scratch. Which if perhaps not a bad thing, considering that ACS and ISE are quite different and some person will eventually have to understand and manage it all - it might as well be done right (re-engineered by a human, rather than by some script, that does garbage in:garbage out)
Having done a few of these myself, I think it's wiser to look at the overall solution and check that you're not dragging a load of technical debt with you into the new ISE platform. it's an opportunity to clean up and make things work the way that fits in with the ISE paradigm.
