Jan-
One way to provide this is via the free Advanced Datagrid (ADG) module.  This module is similar to the REPORTS module.  REPORTS is much newer and has some nice features not found in ADG but the ADG module includes an "Export to Excel" option.  Both modules allow you to specify a SQL query in the module settings.  The module returns the query results in the form of a tabular report (perfect for your needs).
 
I've blogged about the Advanced Datagrid several times:
 
   ADG Part 1:  http://www.eguanasolutions.com/DNN_Blog/EntryID/2.aspx
   ADG Part 2:  http://www.eguanasolutions.com/DNN_Blog/EntryID/5.aspx
   ADG Part 3:  http://www.eguanasolutions.com/DNN_Blog/EntryID/13.aspx
 
A very basic SQL query for returning a list of users and emails (returns all uses from all portals):

   select FirstName, LastName, Email from Users
 
 
So, to get a list of users and email addresses for all portals on a DNN installation, place the free Advanced Datagrid module on a page, use the example query above as your "SQL query" and also select the "Enable Export" option under module settings.  Now when you view the module you'll get a list of user names and emails with a "Download as Excel File" icon at the bottom of the list.
 
This example is just a start.  You'll probably want to refine the SQL query to best suit your needs such as: list users only from certain portals, users who are not administrators, users who are currently "active", etc.
 
 
Hope that helps-
-mamlin