No, you cannot.

You can use the .Net program StyleExtractor written by jexino to extract the styles from the .ppf / .ppi files on your Windows computer:
StyleExtractor thread on PSR Tutorial site

Then you can just copy the styles on to your USB flash drive, and play these styles on your PSR-S970 without installing the expansion packs.

Each expansion pack may come with specific voices / sounds without which the styles may not sound properly. You can install expansion packs one at a time to see if you like any of the styles, mark the sounds / styles you like. Then you can use YEM (Yamaha Expansion Manager) to check and include the ones that you like to generate the final installation pack file in order to fit as much as possible the stuff that you like in to the expansion flash memory.

Thanks,
Paul