You can! Once the account is verified and working w/ 1 alias, exit out and edit the info.
Use notepad to enter your various email aliases (there is no comma in the mail setup):
alias1@mac.com,
alias2@me.com,
alias3@mac.com
Copy the whole string and go back to Settings > Mail, Contacts, Calendar > Alias Acct > address
In the "address" field, paste the string of MM addresses. Back out and launch the Mail app. I turned of my MobileMe so as not to duplicate emails. You will loose Push MM email, but can keep all the other MM features like Find my iPhone, Calendars, Contacts.
Now when you compose email, the list of aliases should show up in the chooser. Hope that helps!