In short, you can’t. But there’s a work around.
Let’s say your company’s primary domain is www.PrimeWidgets.com and this is your primary domain in Google Apps. You’re sister company at www.SuperManufacturing.com will be handling sales generated from your secondary domain in Google Apps, www.mywidget.com. You want emails sent to the sales rep firstname.lastname@example.org as well as email@example.com to forward to firstname.lastname@example.org.
How’s that for a crazy ass setup? Yes I ran into just such a situation and so document the resolution for your purusal.
First, you have to setup a group for email@example.com that forwards to firstname.lastname@example.org.
Now your next thought is to add mywidgets.com as an alias of mywidget.com and be done with it; not so fast. As of this writting you can’t attache an alias domain to a secondary domain in Google Apps.
The fix is to add mywidgets.com as an additional secondary domain and setup an second group email@example.com forwarded to firstname.lastname@example.org.
It’s less that elegant, but will work until Google fixes an obviously, to me, missing feature.