Hi partner,
Marketing List has an field "Targeted At" (Marketing List Member Type) that define its member entity.

The members of a Marketing List can only be one entity, such as Account.
This is why the members is always Accounts when you are building a workflow.
In addition, the marketing list members are actually the customer type entities, such as Account, Contact, so we need to select Account or Contact or other entity when new workflow.
We need to create a workflows for each Member Type. For example, if you have three Marketing List, one whose members are accounts, one whose members are contacts, one whose members are leads, then we need to create three workflows, one entity is account, one entity is contact. one entity is lead.
Here is a on-demand workflow for Account. The To field is filled with the dynamic value of Account.

Activate this workflow.
Open Advanced Find, search for Accounts, select filter to (Related) Marketing Lists Contains Data, then select the Marketing List you want to bulk-create records.

Click Result, then select all records and click Run Workflow. Select the workflow you created.

If Marketing List Member is another entity, then create a workflow for this entity and then search for the record of this entity in Advanced Find, and finally run the on-demand workflow.