How can we use an action from Integration-A in a custom action in Integration-B ?

You wont be able to directly reference anything in Integration-A from code Integration-B. Depending on the use case you can either:

1: Design your custom action to accept the relevant bits as parameters and then use a playbook to call the action from integration-A and pass the results to your custom actions.
2: Copy that original action over so it exists in Integration-B.  The easiest method is typically use the export -> items option (In the IDE page, the button looks like a list icon above the integrations/types selector) on the original action then importing that file into Integration-B