Is it possible to have a default link on all subsites to the main site.

I have set up a multisite marketpress site on the simple market theme, Is it possible to have a default link on all subsites to the main site?

Also unlike the main site, the sub-site menu's seem to be pages by default, how do I add other items or links to them, as I cannot do this in the menu setting as I would in the main site.

Thanks

Dani