I have a flow 'FlowA' registered in prefect cloud. Can I set a schedule for FlowA from another flow FlowB?

technically yes, since you can do it via API call, practically, this is not a common/typical pattern If you're asking if you can trigger flowB from FlowA - absolutely yes - example https://www.prefect.io/guide/blog/flow-of-flows-orchestrating-elt-with-prefect-and-dbt/