Hi,
I don't think you can hook up an XFA form to a text file. You can make a data connection to a spreadsheet or database. However this will not work if the user has Reader, UNLESS the form has been Reader Enabled with LC Reader Extensions ES2 first.
Another option would be to maintain the customer data in an XML file and then occasionally import this into the XFA form.
Both options would need an initial set up and there are examples on the forums. Also see Stefan Cameron's blog that has lots of examples on data connections: http://forms.stefcameron.com/
Good luck,
Niall