In short, no.
You can however include multiple copies of the same channel in your task and effectively sample it faster. One problem with this though is the lack of even spacing between samples on a given channel (you can make it work in some specific cases, e.g. ch1,ch2,ch1,ch3...). Most of the time it's better just to sample at the max aggregate rate (500 kHz on 2 channels for the 6259) and decimate in software. What rates are you hoping to sample at?
Best Regards,
John Passiak