My initial approach, would be to try the patch linked to by Maxime's answer, as it looks promising.
However, a more flexible approach would be to override the importing yourself. This may not be entirely appropriate for complete beginners to programming, but it will on the other hand be able to solve the most complex Feeds scenarios. I've done some fairly advanced imports this way.
First, install Feeds tamper. This module allows you to create temporary targets for arbitrary data. Map the source of you user-reference field onto such a target.
Second, create a module that Feeds will accept plugins from. The generics of implementing Feeds plugins is already well covered in the docs, so there's no need for me to repeat them.
Third, we need to tell the feeds processor do something useful with this data.
This custom processor will do just that. Honestly though, I haven't checked exactly what a user reference array looks like, as I don't know when last I used that, and have no site to check it on.
class UserRefProcessor extends FeedsNodeProcessor {
protected function map(FeedsSource $source, FeedsParserResult $result, $target_item = NULL) {
// Perform all the regular mapping.
$target_item = parent::map($source, $result, $target_item);
$user_reference_target = $parser->getSourceElement($source, $result, 'id_of_temporary_target');
$target_item->user_reference_field = array('und' => array(0 => array('uid' => $user_reference_target)));
}
}
And you should be good to go :)
If the user_reference_field =
line is appropriately adapted, this could work for any type of field.