Difficulties while installing additional packages from .iso (USB stick) after successful stretch install

[sbeck@secure.mailbox.org]

Hi list members,

I have successfully installed stretch from a usb stick onto which I had
downloaded the latest amd64 weekly-build DVD-1.iso and other necessary
init files.
Well, after the install process had finished, I rebooted from harddisk and
everything worked, but when I now try to use apt to install more files
FROM THE DVD.iso on the stick (not over the network), it tells me to
insert the cdrom:[DEBIAN_TESTING...DVD-iso-1], even when the USB
stick is already inserted in the slot - no, apt wants a cdrom (and that's
what the source.list indicates: cdrom. 
It's an iso-hybrid as you know, but I can't find a way to tell apt to use
this very same USB stick (with the named ISO) from which the installing
process had been carried out. 
When I put different forms of identifying the USB stick into the
sources.list apt says something like "there is no driver for that device
installed" or "there is no release file" (when I put
file://var/lib/apt/lists directory) and does not proceed. But the
installer correctly used this USB stick. I can't figure out how to work
around that, or what's going wrong. Do you have an idea?
I read the install.txt from beginning to end, but I couldn't find any
hints on this issue.
 
Thanks in advance.
 
Stephan

P.S. Please CC me as I'm not subscribed to the list.