Dide,
So your external clock that you want to sync your DAQ card with is 10MHz or 20MHz, is that correct? If you want to use an external clock as a sample clock it's pretty straigforward (example here). However that external clock has to be at the rate of your desired acquisition, so it can't be higher than the max sampling rate for the card.
If your clock is at 10MHz then you could use it as a reference clock and then the timebase on the DAQ card will 'lock' to that reference and you would have your synchronization.
Aaron W.
National Instruments
CLA, CTA and CPI