In NAC Manager go tools --> Options --> Data Persistence.
You can set the timer to 0, however this means that every end system that attaches to the system will never be purged, so eventually you'll end up with a large amount of old end systems.
What you can do is as Roland has said put these special end system into a group and make sure the option to "Remove Associated MAC locks and Occurrences in Groups" is NOT checked.
Once the end system ages out and re-authenticates it should authenticate back into it's end system group rule as the option to remove has been disabled.
Also, if you can get RADIUS accounting, or a DHCP packet from these devices it'll reset the last seen time and they'll never age out.
You can also set a session timeout or re-authentication timer on the port to have the device re-authenticate after a period of time, resetting the last seem timer so these devices don't age out either.
Thanks
-Ryan