What is the minimum output frequency according to this datasheet?
The resolution of the DDS is equal to the clock frequency divided by 2^32 since the tuning word is 32 bits long.  For a 125 MHz clock (the maximum allowed) this results in a minimum frequency of 0.029 Hz. If you need lower frequencies, just use a lower speed clock.  The minimum clock frequency specified is 1 MHz.  This would result in a minimum frequency of 0.0002328 Hz which could be increased in steps of the same amount. You could, of course, use any clock frequency between 1 and 125 MHz to suit your requirements.