Product: List Management
Description: This solution discusses manually adding a list record to an existing list
Adding an organization to list management is not currently functionality, however there is a way to convert the record.
Step 1: Once the record is added to the list, right-click and select "Open record".
Step 2: In the open record select Constituent from the toolbar then select "Convert to Organization".
Step 3: You will be prompted to confirm that you would like to convert the record. Save and the record will be added as an organization list member.