rodericks on "[Plugin: WooCommerce - excelling eCommerce] Add extra variable / info to order"

I want to like to add an extra variable to the order. It is a variable from outside WooCommerce, and is loaded with a GET variable. Frontent everything looks good, but I don't know how to add that GET variable to the order.

So my questions: where do I have to store the variable?
And where do I have to place the code that stores the variable.

Thanks!

http://wordpress.org/extend/plugins/woocommerce/