Sorry but there is no easy way to do that.
You may bluff the scope and copy the data of interest at 1GS/s saved file to lower sample rate file that is visible in spectrum analyzer:
- Save the data of interest at 1GS/s to file using "Save DSO Data" option.
- Save any data to another file on lower sample rate (e.g. 1ms/div).
- Copy and paste the data section of the 1GS/s file to this file. (Or copy the header section of the 1ms/div file to the 1GS/s file.)
- Open the file containing 1ms/div header and 1GS/s data.
- Click "Spectrum Analyzer" to see the spectrum.
I'll try to see the result.
Thanks for the trick.