modify magento shipping plugin

I created a plugin for magento "ritiro in negozio" (order online and collect in store), i need in this plugin a simple dropbox for the costumer to choose one of our 5 shops. Selection has to be mandatory and stored somewhere in order details sales_order_grid, possibly appended at column shipping_information. We don't need a control panel to change or add other shops, we will do it by code.