I have also found a program (shareware) that can open, add files, and recompress tar.gz files (especially androidinstall.tar.gz) I have tried using 7-zip, Win-Zip, and Win-Rar to do this but Winrar and Winzip will not allow me to do thos. 7-zip just gives me a memory allocation error.
Normally, you would have to install .apk files one by one which is time consuming but by adding the apk files you want to install to the directory /sdcard/npkinstall/androidinstall.tar.gz/data/apps , you can save time and it will automatically install it when you run install.exe or when you first install a neopeek android port onto your device.
The program name is "PowerArchiver"
<<the official website. It gives you a 30 day trial, sort of like winrar but better
A video to demonstrate the process of installing neopeek builds..