What's the best way to install apt packages from Debian Stretch on Raspbian Jessie?

As backports has letsencrypt, I recommend using jessie-backports as it brings less new/packages dependencies than drinking directly from stretch.

To use Jessie backports and install letsencrypt from it:

Add to /etc/apt/sources.list:

deb http://httpredir.debian.org/debian jessie-backports main contrib non-free

The run:

apt-get update

As for installing the key, I confirm you can do:

gpg --keyserver pgpkeys.mit.edu --recv-key  8B48AD6246925553      
gpg -a --export 8B48AD6246925553 | sudo apt-key add -

and also with the key 7638D0442B90D010

gpg --keyserver pgpkeys.mit.edu --recv-key 7638D0442B90D010     
gpg -a --export 7638D0442B90D010 | sudo apt-key add -

And finally to install letsencrypt:

apt-get install -t jessie-backports letsencrypt

For me, the easiest way was run:

aptitude install debian-keyring debian-archive-keyring

No more errors :)