It's a read-only parameter. That said, there's nothing stopping you from editing the files in $HTS_USER/.hts/tvheadend/input/linuxdvb... wander down that tree through networks and muxes until you get to services, and you'll some service files eventually. Try editing one you know you'll find again/recognise a change, and see what happens... maybe shut down tvh while you do it.
No idea if it'll work, though!