It has a 0-10v dim input. Can I just take a 10k digital pot and dim away?
Probably... But, it would be helpful to know the input impedance (or current).
For example, if the input impedance (of the dimmer circuit) is 1K and you use a 10k pot, the voltage won't be linear (you'll get around 2V at the mid-point, instead of 5V). To some extent, you can compensate for non-linearity in the software if necessary.
And, if the impedeance is really low (very unlikely) you can get excess current through the pot, and damage it.
It wouldn't hurt to try with a regular analog pot. Worst case you fry the pot, but that's very unlikely.
Are digital pots fast enough for "switching" as well?
Probably... What frequency are we talking about? You can check the specs, but I assume they work at audio frtquencies (beyond 20khz).