Can a package pull a another package from another repository to be added?

[adrelanos <adrelanos@riseup.net>]

Hi!

A derivative (Whonix) is based on Debian stable. No Debian source/binary
packages are modified (no need yet). Just some meta and configuration
packages are installed. Those packages receive upgrades from an apt
repository hosted by the derivative.

One of those existing packages, when upgraded in future, should fetch a
package from Debian testing using apt pinning.

When a package gets upgraded by apt-get... There is no way I know of for
the package to:

(1) add a new /etc/apt/sources.list,
(2) add new apt pinning settings to /etc/apt/preferences.d/
(3) update package lists (apt-get update) and
(4) pull a new package from another repository (apt-get install
python-stem - only available in testing, not in stable)

(1) and (2) is simple, but a postinst script can't run (3) "apt-get
update", because apt-get is already locked. Neither it can (4) pull a
new package from a new repository, also because apt-get is already locked.

If the package would depend on the package only in stable in the first
place, it wouldn't install at all, since apt pinning isn't set up yet.
Its a deadlock.

Do you have any idea how to pass the deadlock?

[I hope its on topic here. As far I know debian-mentors list isn't for
non-Debian packaging related questions. Please feel free to redirect me
to a more suited place.]

Cheers,
adrelanos