Hi Greg,
Unfortunately, the DGX-670 (like the CVP models) is NOT compatible with the Yamaha Expansion Manager (YEM) technology. This is probably mainly due to the fact that the DGX-670 requires all the resources of the internal tone generator and the complete internal NAND flash memory (in which the voice's wave data is stored) for the internal voices (especially for the high-quality Piano voices and additional, high-quality effects I think).
The CVP-809 (like the Genos) even contains TWO tone generator chips. However, while on the Genos one of the tone generator chips including the associated NAND flash memory is reserved for Expansion Voices, the CVP-809 uses BOTH tone generators and the complete internal NAND flash memory for the high-quality internal voices.
Only the following models are currently compatible with the YEM Expansion Pack technology: Genos, Tyros5, PSR-SX models, PSR-A5000, PSR-Sx70 models.
P.S.
So you definitely can't install Expansion Packs on the DGX-670, i.e. no new voices (with new samples) can be added. You have to make do with the included voices and the onboard editing options (e.g. Voice Set / Mixer / Effects). .
But you can still use most available Yamaha STYLES on the DGX-670 (directly from the USB stick). Here on psrtutorial.com there are many Styles for free download.
Best regards,
Chris