iWeb is discontinued, so the likelihood that it gets the menus you want is rather slim.
And yes, there is a seamless way to add them.
Start here :
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Well, considering that Apple has withdrawn support for iWeb and did so last year and no longer sells it in any of its stores and has not updated it in over 3 years now, then you will not get drop down menus.
As Apple does not support iWeb any longer, they are not going to update it, so forget the drop down menus now and consider re-building your website in another programme. One such programme that does have drop down menus integrated into it is WebAcappella and you can find it by looking here at http://www.webacapella.fr.
You can create ordinary menu buttons and also a dynamic menu which is a very easy to use drop down menu and it is very easy to use and like iWeb too. Take a look at it and see for yourself, but if you want drop down menus then forget about iWeb.