Free-from relations are now supported, but only in the VCard 4.0 way:
At the moment, only (free-from) “text” values are supported (not supported: email addresses, URIs).
If the server supports VCard 4.0, it should understand that. Please note that when testing, we have found some servers that say to accept VCard 4.0, but all of them had problems handling such relations. Some accepted and processed the RELATED properties correctly, but converted them into X-CHILDREN etc., while other servers just ignored the field.
If the server doesn’t accept VCard 4.0 (you can check in the DAVdroid account settings), RELATED will still be sent but VCard 3.0 doesn’t define RELATED so you can’t expect it to work.
DAVdroid won’t support undocumented X- types for relations. If you have problems with your server, please check whether the VCards are processed by the server correctly and whether your server sends valid version 4.0 VCards before filing bugs.