create final deb file from multiple deb files

It is possible, but you'll likely run in several issues. First, each package has it's own pre- and post-installation scripts and dependencies in the control file. Even if you make a package provide the contained packages using the Provides field, future upgrades may ruin your God package:

1. You install your God package which provides a lot packages
2. One of the packages are being updated, like libpango1.0-0
3. If you upgrade that package, your God package will be removed. If there are any dependencies on it, apt-get has an unresolvable conflict.

So, it's better not to create a God package, but install each package separately. If you need to install software offline, see How can I install software or packages without Internet (offline)?

How to create a God package (not recommended):

1. Create a temporary directory, e.g. "~/godpackage" and cd into it
2. Extract each .deb file using dpkg -x filename.deb .
3. Extract the control, postrm, ..., files using dpkg --control filename.deb tmpdeb. A new directory will be created, named tmpdeb. Adjust the control files like changing the name to avoid conflicts later. When done, move / merge the tmpdeb directory with the DEBIAN directory (create if needed). Repeat it for each deb file
4. Go away from the directory: cd ..
5. Create the new debfile from ~/godpackage and store the newly created .deb file in the current directory: dpkg-deb --build ~/godpackage .