Ben,
The closest you are going to be able to get to what you want is to use a DAQmx acquire with a Start Trigger and a Reference Trigger. When the Start Trigger pulses, the acquisition will start sampling. When the Reference Trigger pulses, you will get a pre-defined number of samples before the Reference Trigger as well as additional samples after. You will have to set Acquisition Mode to N Samples to do this. This is the closest to a start and "stop" trigger you can get in SignalExpress.
The other option is to use Start and Stop Conditions in the Recording Options of the acquisition. This can record data based on hardware, software, or date/time conditions. This will be you're best bet for creating a log of data during particular events.
Regards,
Aaron
National Instruments