Any advice before I completely destroy my costume?

[Kpoet]

I bought a costume for my Latin Halloween party on Saturday assuming from the measurements on the package that it would fit. Wrong. The main piece is a dress that does up in the back with 5 velcro closures. The top 3 come together with room to spare, the 4th one closes perfect and the bottom one doesn't even come close.
I put it on inside out and had my hubby take some measurements for me. At the bottom closure I need 4" for it to fit, tapering up to the next bit of velcro which is roughly 6" above.
Ideally I'd like to take out the velcro and add a zipper but I don't know how to add fabric so the bottom closes. I also marked where the top bits need to be taken in which I can do easily after the zip is in.
Any suggestions?

[OrangeKnickers]

What about making the back lace up?  Add a privacy panel to the back (or not, if you don't care about exposed skin) and lace it up corset style.  That way it can get tighter where you need it and looser where it needs more room.

[Kpoet]

I actually thought about doing that but my experience with grommets has been disasterous. I did end up adding some matching fabric where needed and putting in a zipper but when I put it on and bent over the side seams split. Now I have to figure out how to fix that.
The zipper was my best one ever though!

[OrangeKnickers]

If it turns out that the zipper idea just won't work, you can use button holes instead of grommets.  Just don't try to make the fit too tight or it will bunch up strangely.  Good luck.

[ArabellaCraft]

With grommets you want to use good quality ones (the two-part ones, not the one-part eyelets), and reinforce the area you're installing them with some good interfacing or even canvas.  For a lace-up garment you also want to reinforce the edge with some kind of boning (plastic boning is fine in a pinch, or even plastic zip-ties) or the area will crumple when you lace it up.

[LadyUranus]

If you hate grommets, or are worried about the fabric being too thin, try just sewing around the hole. You'll probably have to do it by hand, but it's a life saver.

You can also use jewelry pliers-- that's the only way I can ever attached the metal grommets.