Can i use a gethttpsforfree to create a valid certificate for locahost or intranet domain?

I have a webapp ruing in a public domain using a ssl certitifcate provided by https://gethttpsforfree.com/

It works like a charm.

The problem is that sometimes i need to install this webapp directly in clients servers in intranet networks (not in my domain) and it need another certificates for a specific hostname in clients intranet.

Since, I don't want to share my real domain private key nor add security exception for each client browser coected, how to solve this problem?

In step 2 of https://gethttpsforfree.com/ I can add others domains to be secured, I think it would resolve the issue, but not sure if by creating a new certificate will invalidate my previous one.

Anyone can give me any tips on how figure out this problem ?