Default Billing Information on PayPal site using WPS
Thanks for your helping clues.
I'm using Drupal Commerce V7.x-1.8 with drupal 7.26 and commerce_paypal WPS 7.x-2.3.

I made a custom module named my_module. Code Below :

function my_module_commerce_paypal_wps_order_form_data_alter(&amp;$data, $order) {
  $user = user_load($order-&gt;uid);
  $profile_id = $order-&gt;commerce_customer_billing['und'][0]['profile_id'];
  $profile = commerce_customer_profile_load($profile_id);

  $data['first_name'] = $profile-&gt;commerce_customer_address['und'][0]['first_name'];
  $data['last_name'] = $profile-&gt;commerce_customer_address['und'][0]['last_name'];
  $data['address1'] = $profile-&gt;commerce_customer_address['und'][0]['thoroughfare'];
  $data['address2'] = $profile-&gt;commerce_customer_address['und'][0]['premise'];
  $data['city'] = $profile-&gt;commerce_customer_address['und'][0]['locality'];
  $data['zip'] = $profile-&gt;commerce_customer_address['und'][0]['postal_code'];
  $data['night_phone_b'] = $profile-&gt;commerce_customer_address['und'][0]['phone_number'];
  $data['email'] = $user-&gt;mail;

}


Now the customer billing informations are automatically filled up in the Paypal Form.
Notice for phone field : I used "night_phone_b" field for European phone format.
I also hope this helps someone else.