Best way to add slave to the XtraDB cluster

Hi guys!
I have an 3 node percona xtradb 5.7 cluster, and, lets say, 3rd party wants to add one more Read-Only server, to play with it and not to influence the production cluster. But, as i saw in documentation, every cluster member acts like a master, so what is the best way to do this?
Mb i can add one more node, and give the replication user permissions only for a read?
Or it is possible to add common MySQL server as a slave to cluster somehow?