Hello Frank,
It is actually called auto-increment. To enable auto-increment in DAQmx you would use a DAQmx Channel Property Node to configure the increment to Load Register A.
The property name is Counter Output->General Properties->Auto Increment Counter->More->Auto Increment Counter.
I still have some trouble picturing you specific case to use it. You can undersample at constant rate.
Best Regards to you as well
Yardov
Gerardo O.
RF SW Engineering R&D
National Instruments