No, it won't let you do that. Adding items from that interface doesn't just add links to the menu, you are telling it to create new pages, and the name that you give to those new pages is used as the filename, which must be unique.
I am guessing that you don't want two new pages (contact, become a member) for each section, but just want menu items that point to those two pages that you already have.
That can't be done from inside the SiteAssist GUI, but can easily be added (I think) by using CSSMenuWriter.
If you have MenuWriter, you should open your template (in the Templates folder) click on the small javascript icon beside the menu, and then click the 'edit contents' link in the properties inspector.
If the small javascript icon isn't showing on your page, then go to Dreamweaver Menu > Edit > Preferences > Invisible Elements > and make sure that the 'scripts' checkbox is ticked.
