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How to"apt update" from an USB key ?

On an ultraslim (ACER swift 3) I have no CDROM no Ethernet, only an USB key.
I have installed stretch (without GUI) from the USB key, and now I want to install connman, but I do not success to apt-cdrom on an USB.
I have googled but did not find a correct howto to do that...

Thanks in advance