This is a message modification module for rsyslog that allows you to change
the facility (and, optionally, severity) of messages. Basic usage is:

  # load module
  module( load="mmpri" )

  # Change facility of the message to 'local0':
  action( type="mmpri" facility="local0" )

Of course, normally the action above will be invoked in a conditional context.
E.g.:

  if $syslogtag == 'myprog:' then {
    action( type="mmpri" facility="local0" )
  }