Your question is a little bit "weird"... You can not add a read-only node to an FCI
FCI = Failover Cluster, only one node is active. This one node can only handle read/write access.
Maybe you are talking about an Availability Group, there you have read-only nodes (readable secondaries)
If you just have an FCI, then one option might be "Log-Shipping" into Azure or "Replication"
If you have an AOAG, then it depends on the network (latency) and the SQL configuration.