jimjung27,
The following is for E9.1/TR9.1. You don't mention what versions you are using.
The System function Send Message Extended does not cater for a "from" email address. The application P07640 does not use the System function Send Message Extended. It uses the Business function SendLeaveApprovals.
Here are some business functions that a quick search found that may be useful(why couldn't you do the same?). They all have provision for a "from" email address. You will have to work out how they are used and which one (if any) suits you best.
Business Functions:
EmailMergeLetter (B0500725)
SendHRMEmail (B0501320)
HRMSendEmail (B0800603)
JDEMSendEMail (B0200900)
I prefer to use the business function ExecuteExternalProgram (B34A1030) and use email functionality on the server.