Yeah, you can send PMs to any group as a whole. For staff, just enter in ‘staff’ in the user field.
You may need to enable it first (I forget what the staff group’s default is) by going to admin/groups, selecting the group, and changing the “who can message and @mention this group” setting.
As you can probably guess from the name of the setting, this also allows you to mention the group in regular posts, just like you would for any user (e.g. @Swedz, so you could use @staff).