To register an existing farsync instance in the clusterware:
srvctl add database -db <db> -oraclehome <home> -startoption mount -spfile +DATA/<db>/spfile<sid>.ora -pwfile +DATA/<db>/orapw<sid>
srvctl add instance -d -<db> -i <sid> -n <node>
To copy the password file from the filesystem, use pwcopy (syntax may varry depending the cluster version)
ASMCMD> pwcopy --dbuniquename <db> /tmp/orapwemdev13c +DATA/<db>/orapw<sid>
copying /tmp/orapw<sid> -> +DATA/<db>/orapw<sid>
As far as the spfile is concerned, the method so far is to use the standard method to copy from dummy pfile.
SQL*Plus: Release 12.1.0.2.0 Production on Tue May 5 10:42:19 2020
Copyright (c) 1982, 2014, Oracle. All rights reserved.
Connected to an idle instance.
SQL> create pfile='/tmp/init' from spfile;
File created.
SQL> create spfile='+DATA/<db>/spfile<sid>.ora' from pfile='/tmp/init';
File created.
SQL>
Then make sure the init file in the ?/dbs directory points to the same spfile:
$ cat $ORACLE_HOME/dbs/init<sid>.ora
SPFILE='+DATA/<db>/spfile<sid>.ora'
The far_sync instance can now be started from the clusterware:
$ srvctl start database -d <db>
$ srvctl status database -d <db>
Instance <sid> is running on node <node>
Be sure also to check for any issue in the alert.log.