<p>I have an XML file that starts like this:</p>

<pre><code>&lt;Elements name="Entities" xmlns="XS-GenerationToolElements"&gt;
</code></pre>

<p>I'll have to open a lot of these files. Each of these have a different namespace but will only have one namespace at a time (I'll never find two namespaces defined in one xml file).</p>

<p>Using XPath I'd like to have an automatic way to add the given namespace to the namespace manager. 
So far, i could only get the namespace by parsing the xml file but I have a XPathNavigator instance and it should have a nice and clean way to get the namespaces, right?</p>

<p>-- OR --</p>

<p>Given that I only have one namespace, somehow make XPath use the only one that is present in the xml, thus avoiding cluttering the code by always appending the namespace.</p>

Unfortunately, XPath doesn't have any concept of "default namespace".  You need to register namespaces with prefixes with the XPath context, and then use those prefixes in your XPath expressions.  It means for very verbose xpath, but it's a basic shortcoming of XPath 1.  Apparently XPath 2 will address this, but that's no use to you right now.
I suggest that you programmatically examine your XML document for the namespace, associate that namespace with a prefix in the XPath context, then use the prefix in the xpath expressions.