como modificar los campos del registro de un cliente en wordpress  

Hola Jaime, 

Reemplaza el contenido del archivo /wp-content/themes/martfury/dokan/global/seller-registration-form.php, por el siguiente:

<?php
/**
* Dokan Seller registration form
*
* @since 2.4
*
* @package dokan
*/
?>


<?php do_action( 'dokan_reg_form_field' ); ?>

<p class="form-row form-group user-role">
<label class="radio woocommerce-form__label-for-checkbox">
<input type="radio" name="role" class="woocommerce-form__input-checkbox" value="customer"<?php checked( $role, 'customer' ); ?>>
<span>
<?php esc_html_e( 'I am a customer', 'martfury' ); ?>
</span>
</label>

<label class="radio woocommerce-form__label-for-checkbox">
<input type="radio" name="role" class="woocommerce-form__input-checkbox" value="seller"<?php checked( $role, 'seller' ); ?>>
<span>
<?php esc_html_e( 'I am a vendor', 'martfury' ); ?>
</span>
</label>
<?php do_action( 'dokan_registration_form_role', $role ); ?>
</p>

<div class="show_if_seller"<?php echo $role_style; ?>>
<?php do_action( 'dokan_seller_registration_field_after' ); ?>

<div class="split-row form-row-wide">
<p class="form-row form-group">
<label for="first-name"><?php esc_html_e( 'First Name', 'martfury' ); ?>
<span class="required">*</span></label>
<input type="text" class="input-text form-control" name="fname" id="first-name" value="<?php if ( ! empty( $postdata['fname'] ) ) {
echo esc_attr( $postdata['fname'] );
} ?>" required="required" />
</p>

<p class="form-row form-group">
<label for="last-name"><?php esc_html_e( 'Last Name', 'martfury' ); ?>
<span class="required">*</span></label>
<input type="text" class="input-text form-control" name="lname" id="last-name" value="<?php if ( ! empty( $postdata['lname'] ) ) {
echo esc_attr( $postdata['lname'] );
} ?>" required="required" />
</p>
</div>

<p class="form-row form-group form-row-wide">
<label for="company-name"><?php esc_html_e( 'Shop Name', 'martfury' ); ?>
<span class="required">*</span></label>
<input type="text" class="input-text form-control" name="shopname" id="company-name" value="<?php if ( ! empty( $postdata['shopname'] ) ) {
echo esc_attr( $postdata['shopname'] );
} ?>" required="required" />
</p>

<p class="form-row form-group form-row-wide">
<label for="seller-url" class="pull-left"><?php esc_html_e( 'Shop URL', 'martfury' ); ?>
<span class="required">*</span></label>
<strong id="url-alart-mgs" class="pull-right"></strong>
<input type="text" class="input-text form-control" name="shopurl" id="seller-url" value="<?php if ( ! empty( $postdata['shopurl'] ) ) {
echo esc_attr( $postdata['shopurl'] );
} ?>" required="required" />
<small><?php echo home_url() . '/' . dokan_get_option( 'custom_store_url', 'dokan_general', 'store' ); ?>/<strong id="url-alart"></strong>
</small>
</p>

<p class="form-row form-group form-row-wide">
<label for="shop-phone"><?php esc_html_e( 'Phone Number', 'martfury' ); ?>
<span class="required">*</span></label>
<input type="text" class="input-text form-control" name="phone" id="shop-phone" value="<?php if ( ! empty( $postdata['phone'] ) ) {
echo esc_attr( $postdata['phone'] );
} ?>" required="required" />
</p>
<?php

$show_toc = dokan_get_option( 'enable_tc_on_reg', 'dokan_general' );

if ( $show_toc == 'on' ) {
$toc_page_id = dokan_get_option( 'reg_tc_page', 'dokan_pages' );
if ( $toc_page_id != -1 ) {
$toc_page_url = get_permalink( $toc_page_id );
?>
<p class="form-row form-group form-row-wide">
<input class="tc_check_box" type="checkbox" id="tc_agree" name="tc_agree" required="required">
<label style="display: inline" for="tc_agree"><?php echo sprintf( __( 'I have read and agree to the <a target="_blank" href="%s">Terms &amp; Conditions</a>.', 'martfury' ), $toc_page_url ); ?></label>
</p>
<?php } ?>
<?php } ?>

</div>

Saludos 🙂