WE have the same question another user had a few months back. PLease see below. But that question was never answered.
"Facebook has recently added the ability for pages to join groups. When I enter a group as myself, I'm presented with the choice join the group as one of my pages. However, the choice is just for a few of my pages and not for all of them. Why is that? or How can i add the pages not showing in "interact as"?"
image and question was taken from the link above as well.