I think it's smart to separate the two. Doing so eliminates a lot of baggage since presuming we exist in a creation does not have to involve the traditional gods. We can ask who or what created us.Only once it was established beyond any doubt that we exist within a creation. Until such - questions of GOD are cart before horse stuff. Can one be agnostic re the question of existing within a creation, or only re the question of creator?
Based on the common definition of agnosticism, which is uncertainty towards God's existence, I would say someone can only be agnostic towards the creator.