It's quite easy to get it signed. When you have a signed add-in it will automatically load, it cannot be blocked anymore.
To do this you have to create some certificates, which you can do for free if it is for internal use. I used http://www.itiverba.com/en/software/. It also has a guide. When you are done use signtool to include the certificate in your add-in. If you then add the certificate to trusted certificates in windows your add-in will be known by windows and inventor.
