As far as I know, there is no way to preview the Yamaha expansion packs.
All the sounds are "locked" so you can't open them for edit inside YEM in order to preview them.
You can export only the styles in order to "preview" them (the installation file will be very small) but most styles from Yamaha expansion packs include sounds form the pack so... even if you try to "preview" them, you will hear the without important sound elements...
So, as far as know, the answer is NO, you can't preview expansion packs for Yamaha.
You have to install them one by one, select the sounds and styles who are valuable for your music and then create a "mixed - personal - best of" expansion pack including only sound and styles useful for your music.
On the other hand, many 3rd party expansion packs contain "unlocked sounds" so if you double click the sound name on YEM, you enter the "edit mode" and there you can "preview" the sound, but this is not possible on Yamaha expansion packs.