Heads up: this is now a first-class feature with new firmware added to the NI 414x SMUs, as well as the NI 4135->4139. The feature is included in NI DCPower 18.2: http://www.ni.com/download/ni-dcpower-18.2/7859/en/ . The relevant attribute is "Sequence Step Delta Time", which as the name implies is a defined time between steps of a sequence reguardless of source delay or measure time.
A sine wave could be created by feeding in the waveform points and a constant "delta time", but for more complex waveforms you may adjust the time between points to provide finer resoltion during interesting parts of the waveform, such as a simulating the startup behavior of a power rail. Check the SMU specifications for minimum delta times and resolutions, they very slightly by product.
Happy waveforming
