I'm on version 8.3.2 P7 so my gui may be different, but can you copy the files onto a machine on the closed network and then under "Maintenance / Software Management" navigate to "Download Packages" and click on "Perform Manual Load Now" and upload the files to the firewall from there? Once uploaded, I would think you'd be able to then Install them.
Again, I'm not sure of your options with 7x so this may not be an option.
I can't remember if the option to load patches directly was present in v7 or if it was introduced in v8.
However, you can upload patches via FTP (given that's one of the mechanisms available when downloading them from the internet). All you need to do is download the patches manually, set up a small FTP server on your closed network, copy the patch files onto this device and then change the FTP settings so that the Firewall performs the FTP download from the IP address of that device, rather than from the internet download location.
Once uploaded to the appliance they can be installed as normal.
We load the patches to our home directory via sftp, then using a ssh session do the following
cf patch load patch=xx.xx source=file dir=/home/username
once loaded, we then install via cli
cf package install package=xx.xx
Remeber to remove the packages from your home directory, otherwise the backups can get large.....