How to add dependencies that exist in another repository

[Reza Alemi <alemi.reza@gmail.com>]

Hello all,
I don't seem to be able to find my answer searching the net, so please
forgive me if my question is too trivial.

I am packaging a product which is dependent on sun-java6-sdk (upstream
says it won't work with openjdk). in ubuntu, that package is in the
partner repository which I can get user consent to add, using debconf.

in shell, I would normally do an add-apt-repository for the repository
and then an apt-get update before I can install the sun-java6-sdk
package.

my question is, can I do something like that in my own install scripts?
can I run add-apt-repository and apt-get update while the package is
being installed? won't it break if the package is installed with
aptitude or dpkg an on debian instead of ubunut?  

Thank you,
Rex