Creating a package using two sources/origins

[address@hidden]

Dear Guix members,

I would like to contribute to the project by creating a package for
linux-xenomai (a real-time extension to the linux kernel, see
https://source.denx.de/Xenomai/xenomai/-/wikis/home).
My issue is that, according to xenomai's build instructions, I am
supposed to use both a mainstream patched kernel (I'm pretty sure that
a deblobbed linux-libre kernel can work too) and a .tar.bz2 source
tree that contains the user-space libraries and binaries but also some
scripts for preparing the kernel before building.

By looking into linux-libre package definition, I see no problem in
downloading and patching the proper kernel source, however I don't
understand how I am supposed to specify in the package definition that
we need an additional source tree in the build process.
What would you suggest? Using snippets or additional build-phases?
Either way, I'm not skilled enough to download a compressed tar and
unpack it in a reachable directory at the build stage. I'm only aware
of the package's "origin" function to accomplish that. What is the
best way to do the same in a snippet or phase?

Thanks for the advice. I need this input to study more the topic ;-)

Regards, Davide