Why do you want to use a textarea field in the post editor and convert it into a repeater field? Why not just use a repeater field?
The reason that I ask is that you’re still going to need the repeater field on the post editor, so this seems like a lot of extra work if the text area and repeater are both attached to the same post.
It would be possible, you would need to have a filter that runs on the acf save post hook: http://www.advancedcustomfields.com/resources/acfsave_post/ and then takes that value and adds values to the repeater, possibly with the update_field and update_sub_field functions in ACF
http://www.advancedcustomfields.com/resources/update_field/
http://www.advancedcustomfields.com/resources/update_sub_field/
