If you are sending internally to Notes users, setting "From" and "Principal" fields to a new name should be enough. (see Agent FAQ lik below)
But that's for beginners.
When hardcore developers want to set a fake sender to external emails, they COPY the document into server's mail.box database instead of sending it. This can done with a LotusScript or Java, preferably in background agents. Several fields must be appended to the document before copying. I think I've seen a complete example of such LotusScript in this forum, otherwise send me an email and I will find my own example.
Agent FAQ mentions 3 ways of changing sender name. In many cases you can use them instead of mailbox solution.
See "How can I change the apparent sender of agent generated mail?" section.