Hi,
I dont think there is need to insert that information into back-end of your site, easiest way to do this is just to pick that info up from original website using php curl library, extract information with SimpleHtmlDom, or with pregmatch. And then when you have that information (if there is stock or no) all is left is to just display it somehow and/or disable "order" button (or something like that, to prevent orders when there is not items in stock) based on information of stock quantity.
This is my solution to this problem, script can also be made flexible so its easy to customize it to fit different websites that are products from.
If you have any questions contact me via pm, e-mail or skype, skype id: live:kristijan.sekeres_1
Kind Regards,
Kristijan S.