Update install repository on liveUSB?

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http://unix.stackexchange.com – I'm using a kubuntu liveUSB with persistence. It's possible to update the liveUSB itself, such that the updates persist. However, the update packages aren't used for installation, as far as I can tell. Is there any way of updating the packages on the install image, such that on the next install, those packages don't need to be re-downloaded? (HowTos)