how to set a user-defined process as the system key process

Your reqirement can be achieved by setting your process as a system process (ESystemCritical) or permernant process (ESystemPermanent) by User::SetCritical() or User::SetProcessCritical(), but note that the API requires ProtServ capability.

"Stopping critical phone service"
http://developer.symbian.org/forum/s...ead.php?t=2837

"Developer Library: ESystemCritical/ESystemPermanent"
http://developer.symbian.org/main/do...UserClass.html

"Symbian OS Internals: Types of Symbian OS thread"
http://developer.symbian.org/wiki/in..._and_Libraries