Greetings, Arnold. This is absolutely possible, but as mentioned above, there are many, many ways to handle this. I would begin by creating a simple template with just your navigation items in it. This way, you can embed that in other templates, making it easy to update down the road.
For allowing teh end-user to add a sub-navigation item, I would personally create a weblog for those navigation items, with each entry being a navigation item. For instance, I have a weblog for Site Information. In that I have these entries:
* About me
* About the site
* Privacy Policy
I can output on my page the full information for one. But, I can also create a drop-down using this code:
<ul>
{exp:weblog:entries weblog="info" limit="10"}
<li><a href="{comment_url_title_auto_path}">{title}</a></li>
{/exp:weblog:entries}
</ul>
What this would do is link to those entries, and if your client added a new entry in this info weblog, it would automatically be added to your navigation. You can use the Pages Module as well if you want to have a special URI for these pages.
So, as you can see, this is entirely possible. If you do want more specific help with implementing this, I’d recommend downloading and trying EE and starting to build out your site, then posting in the How To forum where the community can assist you in building the navigation you want for your site. =)