The easiest way to update your RetroPie collection without internet is the USB method:
A "full collection" of games is useless without the system BIOS. RetroPie emulators require these system files to boot. An updated collection guide must include this: retropie roms full collection of games updated
Looking for a complete Retropie ROM collection? Here's an updated, organized list and quick setup tips to get you playing the classics fast. The easiest way to update your RetroPie collection