You can only combine samples within a single HWS file. HWS does not support the concatenation of signals in a digital waveform datatype.
It is easy in LabVIEW to do this before saving your data to HWS. There is a built in digital waveform datatype VI called "Append Digital Signals.vi". This VI takes two waveforms as input, and concatenates the signals together into a new waveform. For instance, if waveform A had 4 signals and 1000 samples, and waveform B had 12 signals with 1000 samples, the resulting waveform will be 16 signals with 1000 samples. I think this is what you are looking for.
To find this VI in the palettes, go to the Waveforms palette, then click the Digital Waveform palette. The VI will be in the Digital Waveform palette.