RE: Adding Leads to a marketing list
Hi,
I’d like to recommend you to create a workflow to achieve your request.
Here is a managed solution which you can import into your environment to solve the problem that there is no OOB method to import contacts into the Marketing List via workflow.
https://github.com/demianrasko/Dynamics-365-Workflow-Tools
Then import the list of contacts into the Contact entity.
Create a workflow targeting at Contact entity and starting when a record is created.

Add the step to add the contacts into a certain existing marketing list.


Of course, you can set a condition before the above step. For example, only certain fields of contact meet certain criteria to be added to this marketing list.
Remember to deactivate this workflow after importing if you don't set the conditions, otherwise it will add any contacts you create later to this marketing list.
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya