If your mail client has other email accounts defined, then you should be able to choose any one of these accounts. Otherwise, Winston is correct, you can't just enter any email address you want. It has to be one defined in the client.
Even so you can't send using iCloud's outgoing server with anything other than your iCloud address as the 'From' address. Of course in Mail on a Mac you can always send with another account's outgoing server, but on the iCloud website mail your are stuck with the iCloud server and its limitation.