Global product categories

I am building a multisite website. I have several vendors accessing the platform, but each and every vendor needs to place its products in the same sort of categories.
So in the backoffice's Products page, i would need a metabox to select the product's category (but i want it to be a GLOBAL category). There also need to be a form to manage these global product categories. It seems to me this form should be located in the "network admin" area.

Any idea how i could achieve that ?
I have thought of solutions with bbAggregate and post-index, or sitewide-tags, but neither of them really solve my problem.

Thanks in advance!