For example, if your Mage project is named demo_project and your existing dbt project is named demo, add your dbt project in the Mage project under the following path: 
demo_project/dbt/demo/
You can have multiple dbt projects in a single Mage project under the folder named dbt/ at the root location of your Mage project. For example: 
demo_project/
|   dbt/
|   |   demo/
|   |   |   ...
|   |   project_2/
|   |   |   ...

​
Required structure

​
Connection profiles

For Mage to properly run dbt commands, you must have a profiles.yml file at the root of each of your dbt projects. For example: 
demo_project/
|   dbt/
|   |   demo/
|   |   |   profiles.yml
|   |   project_2/
|   |   |   profiles.yml
For more information on how to configure dbt connection profiles, read this documentation.