You could match using the HTTP_HOST
condition
<rule name="RedirectToFoundation" stopProcessing="true">
<match url=".*" ignoreCase="true" />
<conditions>
<add input="{HTTP_HOST}" pattern="mysitefoundation.com" />
</conditions>
<action type="Redirect" url="http://www.mysite.com/foundation/{R:0}" redirectType="Permanent" />
</rule>