WordPress MU

I develop a plugin with custom fields.

Look at
wp-admin/user-new.php
Line 76
wpmu_signup_user( $new_user_login, $_REQUEST[ 'email' ], array( 'add_to_blog' => $wpdb->blogid, 'new_role' => $_REQUEST[ 'role' ] ) );
I think it should be
wpmu_signup_user( $new_user_login, $_REQUEST[ 'email' ], apply_filters("add_signup_meta", array( 'add_to_blog' => $wpdb->blogid, 'new_role' => $_REQUEST[ 'role' ] )) );

The signup meta is only used for users going through the registration process, not for creating users from the admin area. Adding that filter would not accomplish anything.

A hook that you could use in your plugin is wpmu_new_user.