While the PCI-5112 is certainly capable of phase-locking to a 10MHz clock on its RTSI connector for synchronization, it does not, however, accept an external clock as a sample timebase. The 5112's analog to digital converters always run at 100MS/s and then depending upon which sample rate you've specified, an internal digital circuit decimates the data and then stores the results into memory. In short, the 5112 can be triggered to acquire data from an external source, but cannot be clocked.
However, our newer high-speed digitizers, the PCI-5122 (100 MS/s) and PCI-5124 (200 MS/s) do in fact accept external clocks. If this is a feature that you desire, I highly suggest that you take a look at these devices. I've included a link to their product information pages below.
I hope that this helps to answer your question. Even more details about the 5112's triggering and clocking behaviors can be found in the NI High-Speed Digitizers Help, which I've also linked below. Please let me know if you have further questions.PCI-5122PCI-5124NI High-Speed Digitizers Help