I have a system in Gmail where I forward all mail to one account and enabled an option to choose 'from' address.
Apple is making it hard to set up these email aliases on iOS devices.
So far I've been using method of having multiple accounts set up on iOS but today (after hours of searching) I've found another method. But at a cost.
This method doesn't work with Exchange, so there will be no push.
You can still use Exchange Gmail for Calendars and Contacts though.
You have to create a new account using the option 'Other'.
In the 'Email' field you write all f your addresses that you want to send mail from separated with comma and a space. That's all. But since the keyboard that pops up is meant only for email input you have to write the string in some other field and then copy it back (I suggest Description just below Email).
It should look like this:
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
It's nice that it respects to which of your email addresses the email was send and sets the correct from address on reply.
If you want even more detailed step by step walktrough you can find it here: Solved: Gmail, iPad, iPhone, and multiple from addresses