update user's profile field from another form
You have to modify your form in a hook_form_alter : and the linked_in url will appear in the event registration form

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id){
   if ($form_id == 'eventregistration_node_form'){

      // Find the user obj
      global $user;
      // Load all profile fields
      $account = user_load($user-&gt;uid);
      // Add the linked_in url in the event registration form
      // Supposing your field madhine name is ; linkedin_url
      // Supposing you use the link module for this field (if not, change ['url'] to ['value']
      $form['linkedin_url']['und'][0]['url']['#default_value'] = $user-&gt;linkedin_url['und'][0]['url'];
   }
}


If you don't want to expose the linked_in field during the form do this :

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id){
   if ($form_id == 'eventregistration_node_form'){
     // Hide your field
     $form['linkedin_url'][und]['#access']  = false;
     //Add another submit function
      $form['actions']['submit']['#validate'][] = 'mymodule_eventregistration_validate_form_alter';
   }
}

// Your submit function
function mymodule_eventregistration_validate_form_alter($form, $form_alter){

     // Find the user obj
      global $user;
      // Load all profile fields
      $account = user_load($user-&gt;uid);
      // Add the linked_in url in the event registration form
      // Supposing your field madhine name is ; linkedin_url
      // Supposing you use the link module for this field (if not, change ['url'] to ['value']
      $linkedin_url = $user-&gt;linkedin_url['und'][0]['url'];

    form_set_value($form['linkedin_url'], array('und' =&gt; array(0 =&gt; array('url' =&gt; $linkedin_url))), $form_state);
}