Scott Basgaard on "[Plugin: WooCommerce - excelling eCommerce] Auto populate value fields in checkout billing/shipping area"

Take a look at:

http://docs.woothemes.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/

You can filter these fields.

For example:

// Hook in
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

// Our hooked in function - $fields is passed via the filter!
function custom_override_checkout_fields( $fields ) {
     $fields['billing']['billing_first_name']['default'] = 'test';
     return $fields;
}

That should help point you in the right direction.

Hope this helps, cheers!
Scott