I have successfully implemented a mail-merge system using an Excel spreadsheet as the data-source and a number of different documents/letters that can link to this to create the different documents when required.

I am trying to incorporate a password-protected document into this system. The document, outside of mail-merge, is setup with a password to open the template. Once the template is manually completed and saved as the completed document, through 'save as', the new document continues to be password protected.
I have incorporated this same document into mail-merge. Once the mail-merge is completed a new document is created with the completed template and the password-protection has disappeared.

Is there any way I can keep this document password protected?
(For information the document needs to be secure as it contains customer data which is being emailed to an external company. These documents are currently used within a department of 40 and to keep things simple the requirement is for the password to be automatically set.)

Thank you in advance for any thoughts/advice