64 bit - How to add flash drive to apt get?

I have Ubuntu 15.10 on flash drive and need to get some files from mode.

I did sudo mount /dev/sdb1 /mnt to connect the usb drive.

How do I get apt-get to know where my Ubuntu files are?

I have a Packages.gz I would like to use in mnt/dists/wily/main/binary-amd64