(This answer has been updated from the original which suggested hacking Joomla code, to this much better solution proposed in the comment below - thanks to @Irata!)
Joomla includes the custom fields by means of a plugin which is triggered on onContentPrepareForm. This solution involves writing a second plugin, also triggered on onContentPrepareForm, which ...
Several pointers/recommendations regarding your resolving code for you and future researchers.
implode() has a default "glue" of an empty space. PHP Manual When you want to join array elements into a string with no delimiter, you can omit the glue parameter and solely provide the array.
$nMail = implode($arr);
$nMail = implode([$myobject,'@domain....
So could not get it to work with "Juser", so i just used the code below. This cleaned it up and everything works right. :)
function nUser_joomla($nUser, $myuuid, $myobject, $mypassword, $joomla_group)
$arr = array($myobject,'@domain.com');
$nMail = implode("",$arr);
$db = JFactory::getDbo();
$pass = ...