Unless you do some wefting or modifying, it likely ain't going to work. If you bought from eBay, in order to get wefts you'll have to buy a second wig to salvage wefts from, which is pricey, unless the store you bought from happens to carry wefts.
If you bought from Arda or Epic Cosplay, they sell separate wefts you can use.
If you don't feel like doing any of this, you're unfortunately going to need to buy a new wig. There are some at Arda/Epic Cosplay that are already in ponytails and were specially wefted to be that way.
I personally think this on from Epic Cosplay would work nicely, or the one Ani suggested.
ACen 2014 cosplay: Lara Croft, Erio Touwa, Shirahoshi Hime, Sengoku Nadeko
Otakon 2014 cosplay: Makise Kurisu, Shirahoshi Hime, Sengoku Nadeko
ALA 2015 cosplay: Shirahoshi Hime, Akemi Homura [Haregi ver.], Mayoi Hachikuji
Possible 2015 conventions: Sakura-con, A-Kon