It is MUCH easier to rebrew the base beer and add the wine to secondary, keg, or bottling bucket.
Going from bottle to bulk (carboy, bucket, etc) back to bottle would be a HUGE PITA and most likely result in excessive oxygen pick up.
Re-brewing also allows you to fine-tune your recipe for the wine addition.
Another option is just to blend in the glass. If anything, it will allow you to test out blend volumes and taste before you commit to an entire batch.
Let us know how it turns out! Adding wine (as opposed to wine grapes) is something that is on my to-do list and will (hopefully) be part of my NHC talk this year.