Embroidering something that is already stitched is not easy. Embroidery is generally done with the fabric held taut in a frame.
Whether or not it can be done really depends on what kind of embroidery and pattern you want. The most likely way will be to use a small frame and just frame a small section being worked on and then repeat. But this doesn't really work for dense, continuous patterns.
You'll definitely be able to find western style embroiderers but to be honest, their style of embroidery is not difficult... I can and have done a lot myself. If you are talking about desi work with dabka and other metals, then I think it's unlikely and the fabric will certainly need to be framed, so dunno how that would work if it's already stitched.
I don't think you can get it done here. Most Asian bridal shops have the outfits made in India or Pakistan, you could ask them if they're willing to send it back there for you to get embroidered.