Yamaha doesn't make "expansion packs" per se for their DGX/YPG and PSR-E/YPT keyboards, as they normally use that term to refer to packs which include additional voices along with styles that use those new voices.
However, you can play "external" style files that are stored on a USB drive, as described on page 76 of the DGX-650's
Owner's Manual. The highest-quality style files are those which Yamaha creates, because they know all the secrets about the capabilities and limitations of their keyboards and voices-- not to mention of the style file format itself-- so they know how to get the best-sounding results. However, some excellent style files have also been created by various third parties-- companies other than Yamaha, or talented and skilled individuals who have mastered the art of style-making. Aside from those sources, there are a large number of style files which have been created either by converting styles from one keyboard model to another, or by assembling new styles from parts of existing styles. (Yamaha's higher models include a "style assembly" feature that's not found on their lower models.)
Style files which have been created by Yamaha or their affiliates can be purchased from the Yamaha MusicSoft web site (
https://www.yamahamusicsoft.com/). Be sure to select your keyboard to see the style files which are compatible with that particular model.
You can also purchase style files from other web sites-- just search for "Yamaha style files" to find other vendors who create and sell style files for Yamaha keyboards-- including off of eBay. However, I must warn you that many of the style file collections being sold off of eBay are said to contain style files available for free elsewhere, so bear that in mind before you buy any style file collections off of eBay. An excellent source for free style files is the PSR Tutorial web site (
http://www.psrtutorial.com/). Another excellent resource for locating style files (for free or otherwise) is Jørgen Sørensen's web site (
http://www.jososoft.dk/yamaha/stylesites.htm).
Note that you aren't necessarily limited to style files which were specifically created for the DGX-650, as many style files will play on just about any XG-compatible model (although technically the DGX-650 is XGlite-compatible). There are even some style files that will play on any GM-compatible Yamaha keyboard, although GM voices are more limited in quality and variety than XG voices. Furthermore, there are free utility programs available for converting SFF2 style files to SFF1 style files (although the DGX-650 can use either type), for "revoicing" style files to change the voices they use, for reorganizing the sections of style files if desired, or even for assembling your own style files from other MIDI files-- see Jørgen Sørensen's web site for more details (
http://www.jososoft.dk/yamaha/software/software.htm).
