Cachets are added to uncacheted FDC's all the time. They are referred to as "add-on," meaning the cachet is not contemporaneous with the issue.
There's nothing unethical about adding a cachet. The only issue is when someone tries to either mimic a scarce cachet and pass it off as genuine or tries to insist the add-on is contemporary to the FDC.
If you wish to create your own cachet, the market will determine whether you have created something someone wants to collect.
I have a very nicely hand-painted set of FDC's for the National Parks, issued in 1934, which were hand-painted in 2016 for the centennial of the National Parks. Below is an example:

When I purchased the set, for $60.00, I knew what I was getting and that it was an add-on cachet. I liked the artwork and the way it meshed with the design of the stamp, so I purchased it.
Hope this helps.