There is no way to change the domain name you use with your Google Apps account.
Yes, you can delete your @mysite.com Apps account, but all files and email will be lost. I do NOT suggest taking this drastic step.
No, you will not be able to sign up for a new Free Google Apps account, even if you do make the @mysite.com domain available again.
The only way to get more users in the @mysite.com Apps account is to upgrade to a paid account. (There are many new features available to paid accounts)
Yes, you can "transfer" your users from @mysite.com to @mysite2.com if you are willing to use that domain. Email and Docs can be "transferred" too. Roughly export then import (via POP3 method). This sounds easy in principal but it is MUCH harder in practice.
Example: To "transfer" a single Google Doc from one domain to another I took these steps:
- Share the doc with a user in the destination domain, include edit permissions
- Have that user open the document in the destination Apps account
- Make a copy of the doc under the new domain, optionally copy the shared users too.
- Realize this process will not work for the 100's of existing docs that have built up
Try adding @mysite.com as a sub organization in Users in your second Admin Control Panel. If you can do that you might be able to test some of the things I mention above and it might be easier.
Let us know what you decide and how it turns out.