I think you simply can't. If I am not mistaken the hosted version is just 'as is'. No custom modules can be added

EDIT: I assume you mean an saas version. In case you have your own server, that is hosted by someone else but you have root-acces to it would work just like on a local server and the flow would be as John mentioned below

Create your mod. Add it to the addon directory in your install files on your server. Go to your OpenERP instance > settings > update app list. It should pick up your mod.