Oh, it's my bad. I figured out this method isn't implemented in 7.0. I've backported it from 7.1 and everything works now. It's pretty easy to do that even for 6.3 .. 6.5. You just need to put AddRelated.php (from 7.1 sources) to /include/Webservices/ and execute one of the SQL queries below:
For 7.0:
INSERT INTO `vtiger_ws_operation` VALUES (37,'add_related','include/Webservices/AddRelated.php','vtws_add_related','POST',0);
INSERT INTO `vtiger_ws_operation_parameters` VALUES (37,'sourceRecordId','string',1),(37,'relatedRecordId','string',2),(37,'relationIdLabel','string',3);
For 6.3 .. 6.5:
INSERT INTO `vtiger_ws_operation` VALUES (34,'relatedtypes','include/Webservices/RelatedTypes.php','vtws_relatedtypes','GET',0),(35,'retrieve_related','include/Webservices/RetrieveRelated.php','vtws_retrieve_related','GET',0),(36,'query_related','include/Webservices/QueryRelated.php','vtws_query_related','GET',0),(37,'add_related','include/Webservices/AddRelated.php','vtws_add_related','POST',0);
INSERT INTO `vtiger_ws_operation_parameters` VALUES (34,'elementType','string',1),(35,'id','string',1),(35,'relatedLabel','string',3),(35,'relatedType','string',2),(36,'id','string',2),(36,'query','string',1),(36,'relatedLabel','string',3),(37,'sourceRecordId','string',1),(37,'relatedRecordId','string',2),(37,'relationIdLabel','string',3);