Does Ajani's Chosen allow adding creatures from Enchantments played before this card?
No, it can't do anything about enchantments played before the card comes into play. Triggered abilities don't activate retroactively as if the permanent was there all along &mdash; it missed out, the opportunity has passed, and nothing happens.

Ajani's Chosen was not there to see previous events, so it can't do anything about them. You will not get creature tokens for enchantments that have already entered the battlefield, nor be able to attach them to tokens you create.