Hi Nadine,
You could refer to following solution:
1. Create a custom entity "Temp Contact" with necessary fields such as first name, last name and email.
2. Import those employees as records of the custom entity.
3. Copy "HTML output" of your newsletter.

4. Select a record of "Temp Contact", create a flow and trigger it when "a record is selected".


5. Actions of the flow:
When a record is selected, send an email to its mailbox.

Switch the view of email to HTML mode, paste the copied code to email body.

Result:

Notes:
1. The newsletter is only a HTML email, so please remove links such as "View in browser" and "Unsubscribe".
2. It is actually activity email instead of marketing email, but the email looks like a marketing email.
3. Dynamics Marketing can only track activities of the real contacts(Contact entity), if you want to launch further marketing campaigns on those employees, you still need to convert them to contact records.
Regards,
Clofly