First, determine where your Rocket.Chat instance stores its data. By default, Rocket.Chat uses MongoDB. You'll need access to the MongoDB instance where Rocket.Chat is hosted. Obtain the necessary credentials and connection details to connect to this database.

Use MongoDB's `mongoexport` tool to extract data from the Rocket.Chat database. You can export data in JSON or CSV format. For example, to export messages, run a command like:
```
mongoexport --db rocketchat --collection messages --out messages.json
```
This command exports the messages collection to a JSON file named `messages.json`.

Once you have the data in JSON or CSV format, you might need to preprocess it to ensure compatibility with DuckDB. This may involve cleaning the data, flattening nested JSON structures, or converting data types to match DuckDB's requirements.

If you haven't already, install DuckDB on your system. You can download it from the DuckDB website or use a package manager like `pip` for Python:
```
pip install duckdb
```

With DuckDB installed, create a new database file where you will store the Rocket.Chat data. You can do this using the DuckDB CLI or programmatically with a script. For example, to create a database named `chat_data.db`, use:
```python
import duckdb
con = duckdb.connect('chat_data.db')
```

Use DuckDB's capabilities to import data from the prepared file into the newly created database. DuckDB can read directly from CSV and JSON files. For example, to import a CSV file:
```python
con.execute("CREATE TABLE messages AS SELECT * FROM read_csv_auto('messages.csv')")
```
This command creates a `messages` table and populates it with data from `messages.csv`.

Finally, verify that the data has been successfully imported into DuckDB. Use SQL queries to inspect the data and ensure its integrity. For example:
```python
result = con.execute("SELECT COUNT(*) FROM messages").fetchall()
print(result)
```
This query counts the number of records in the `messages` table to confirm the import was successful.

By following these steps, you can efficiently transfer data from Rocket.Chat to DuckDB without relying on third-party connectors or integrations.