Hi CAME18_LDO,
To install Support Packages and Optional Features like the 'FMU Builder' add-on on a computer that is offline or that is behind a Proxy/Firewall, you will need to use the stand-alone downloader on a computer that has internet access to download the necessary files. After downloading, you can transfer these files to the offline computer for installation.
You can follow the steps provided in the MATLAB Central discussion link below for the detailed instructions:-