Sending email from iCloud with a different Reply-to field
I don't know if this works for iCloud account but it works for regular email accounts, try the following.

Go to Settings > Mail, Contacts, Calendars and then choose the iCloud mail account. If there is account information showing the Address as xxxx@me.com simply edit the field to be something like xxxx@me.com,myname@pobox.com

When you send an email tap on the Cc/Bcc,From: field, then click on the From: field. You should be presented with a listing of email address to choose.