If i understand correctly PDF creator will do the job for you.
It's a virtual printer that "prints" PDF files.
It has an option to add a specified PDF automatically to the background of every PDF printed.
The pre made design for the background: letterhead, sales invoice etc. needs to be in PDF format.
(You can use PDF creator to create these)
PDF Creator website
Download & install
the free version (it's tricky, but you can refuse the 'free' extras)
or the 'plus' version with no ads & better support (currently about $5).
This will install a virtual printer.
Then in programs menu choose the PDF creator folder & click 'PDF creator' to access
The settings window.
Click the 'Profile Settings' button.
On the 'Actions' tab tick the 'Add Background(Only for pdf)' Action,
then click on the text of the item to see the options on the right.
Choose the background pdf file you have previously prepared.
Options (from the help) are:
In case the background file has less pages than the actual document
you can select a Repetition:
No Repetition: The first background pages will be set. Page numbers above the number of background pages do not get a background.
Repeat all pages: The background pages repeat themselves until the end of the document.
Repeat last page: The first background pages will be set. The last background page will be repeated until the end of the document.
Add background to cover: Add the background to cover pages as well.
Add background to attachment: Add the background to attachment pages as well.
There are lots of other options. eg auto save to a folder, whether or not to preview after print, filename prefixes/postfixes etc. see the help.
You can then use it to print from any application and the background you specified will automatically added
Hope that works for you.
(Edit) You can also set up multiple printers,(one for each background)