Well i think it is a bit confusing...

lets break this down.

You want to create a Skin Control that points to another page within your DNN setup? It appears you already have the skin control working.

Next you want to have a Custom Call to NavigateURL where you are giving it just the name and have it create a link to ?some other control within your site?

Off the top of my head I don't recall how to do just the Keyword call for NavigateURL.  But I think what you want can be done through the SiteUrls.config - which is in the root of your DNN installation.   Edit that file have RegisterCustomer.aspx point to the tab you want and your skin control can navigate to that URL.

Hopefully my help was less confusing than your question.