Hi Poppy10,
You could take method below as reference:
1. Open Advanced Settings:

2. Navigate to Settings > Data Management > Templates for Data Import, download Contact Import Template.

3. Copy contact records from your original marketing list excel file into template file, set any column(field) as flag to identify imported contacts.
e.g: I set Job Title as flag field.

4. Settings > Data Management > Imports, import new contacts into Dynamics.
5. Find the imported contacts by Advanced filters and flag field.
(Advanced filters: funnel icon at top right of entitylist)

6. Add those contacts into a specific marketing list in bulk

7. After those contacts have been added to a list, create an on-demand workflow to clean value in flag field.




Regards,
Clofly