Dialogflow how to merge couple of agents in one

You can import prebuilt agents into your existing agent to expand your agent's ability to respond to a variety of inputs. Importing will add intents and entities to your existing agent, expanding its functionality and will not remove or alter any existing intents, entities or functions like small talk.

If you have another Dialogflow agent you'd like to "merge" into another agent you can export one agent and import it into another to "merge" the two agent's functionalities. Please note that this may result in unexpected behavior and is not recommended as a development technique.

Import prebuilt agent says:

Importing a Prebuilt Agent creates a new agent. If you want to include Prebuilt Agent functionality into an existing agent, you'll need to move the intents from the newly created Prebuilt Agent into your existing agent.

So, create a new project, select and move all the intents created in the new agent to the agent you require.