I'm afraid you cannot do that. The user ID is used in several interconnected pieces of information that together make up a ticket. Changing the ownership isn't as simple as editing a field. It's also dangerous; if you were to accidentally change the ownership to a member of the support staff who had already replied to the ticket trying to change back the ownership would have some very interesting effects. There are many more reasons why such a feature is not implemented.
Since you have to practically enter the tickets in the system manually why don't you simply copy the text of the email into the ticket system and manually assign the correct user? It sounds much more straightforward.
Nicholas K. Dionysopoulos
Lead Developer and Director
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