If you're happy editing the .xml file by hand look for the section that starts
Code:
<software>
<kernel>kernel-smp</kernel>
<packages config:type="list">
then add in the package name as for the other packages you'll find there.
You'll also need to make sure the rpm is in a repository that AutoYaST looks at. You can specify repositories in the .xml file like so - you may have such a section already.
Code:
<add-on>
<add_on_products config:type="list">
<listentry>
<media_url>http://123.456.123.456/some/path</media_url>
<product>fooo</product>
<product_dir>/</product_dir>
</listentry>
</add_on_products>
</add-on>
If you want to use a GUI tool you can edit the AutoYast file using the Autoinstallation module in YaST - you'll find it in the Miscellaneous section. That's actually harder to explain than hacking it by hand but there are plenty of guides out there.