Install package as dependency of another package

Question

Suppose you want to install Package X, which depends on Package Y. You also want to install Package Z, which augments Package X, but is not a dependency of Package X. You run:
pacman -S X Z
Now suppose that you no longer want Package X on your system. You run:
pacman -R X
Package X is removed. Now you can run:
pacman -Qdt
To clean up packages orphaned by Package X. Provided nothing else depends on Package Y, Package Y will be removed.
All is well and good, except Package Z was not orphaned by Package X. Even though Package Z is (in this hypothetical example) useless without Package X, it won't be removed automatically because it was installed with pacman -S.
So... is there a way to install Package Z as a dependency of Package X such that, when Package X is removed, Package Z will be orphaned if no other packages depend on it?

Answer 1

Thank you, rwd, that does the trick. Editing the PKGBUILD is quite reasonable when Package X is in AUR, and for official packages, editing /var/lib/pacman/local/X/depends (not Z/depends) works. The downside is that I have to remember that I want Package Z to behave as a dependency of Package X and tweak Package X every time it is updated. So yes, an --asdepof option would be really swell.
For venky80 and anyone else who may be interested in this, here's a real-world example. Suppose you want to install loudmouth for use with amarok. If you later decide to uninstall amarok, you would like for loudmouth to be automatically uninstalled as well (unless some other package depends on it). loudmouth is not a depedency of amarok (incidentally it is an optional dependency, but this will work with any package that is not a dependency).
1. Install amarok
$ pacman -S amarok
2. Install loudmouth with --asdeps
$ pacman -S --asdeps loudmouth
3. Edit amarok's depends file to make it depend on loudmouth
3a. Open the file in a text editor
$ nano /var/lib/pacman/local/amarok*/depends
3b. Add "loudmouth" under the DEPENDS section; e.g.
%DEPENDS%
loudmouth
mysql>=5.1.34
kdebase-runtime>=4.2
taglib
3c. If loudmouth is an optional dependency of amarok (and it is), remove it from the OPTDEPENDS section.
3d. Save and close the depends file.
4. Test to see if it worked
$ pacman -Qtd
$ # loudmouth should not be listed
$ pacman -Rs amarok
$ # loudmouth should be listed