Due to a merge, we have domain A under one Google Apps acccount, domain B under another one. I can create accounts for all the users in domain B to domain A to start a migration afterward. Adding aliasses with domains under account A is no problem.
However, sending email as the account under domain B (which users can request manually with their account, the account in domain B will then be mailed with the question whether they want to allow it) doesn't seem possible with the Admin SDK:
Any email address which exists as a user or an alias on the hosted domain.
Is there any way to start the process users would enter into when manually requesting to be allowed to send mail as that other user / email address?
Migrating the domain to it first is out of the question as apparently, a domain has to be completely shut down before it is allowed to be added to another Google Apps account.
It is not possible to configure a "send as" via API if the original sender belongs to another domain. I cannot see a "click to complete" way for the users. A quick guide for them or some web scraping / Selenium could help you.
Maybe, if it were a domain alias or a subdomain instead of a different Google Apps account, you could.
Have you contacted Google Apps Support regarding this issue?