Default permission level

Hello, firstly I am sorry my English, so I'll try to be as brief as possible. (I work locally (Mov'AMP) with Joomla 1.5.23 and VirtueMart 1.1.8
My problem is: for the moment when a person creates a member account on my site without creating a customer account in virtuemart it is possible to select "user list" then click Next click on the user concerned and then " Customer Information ", the default permission is" select "and you have several choices. I can manually put" buyer ", thus avoiding the fact that the member must create an account. But I wish he begins with" buyer "by default, so that a member automatically becomes the" buyer "do you think it is a way by editing the PHP files corresponding to this question. I tried, but abandoned for fear of a" rupture " .
A big thank you to everyone, if this detail is resolved, I hope it will help others.

Aymon.