I checked my PSR-E443, which is nearly identical to the PSR-E453 except for the new features that were added to the PSR-E453.
There is no way to get the left-hand or "Split" voice to sustain using the pedal.
As Ray said, the accompaniment will keep playing in the last key/chord type used when you lift your left hand, as long as the "Sync Start" option is turned on.
If you don't want all of the accompaniment parts to play, you can use the "Track Control" buttons to toggle the various parts on or off (i.e., "Drums," "Bass," "Chord1," "Chord2," "Pad," and "Phrases"). This can be used to create some interesting variations of the accompaniments. However, even if you turn off everything except for "Chord1" and "Chord2," those two parts usually play arpeggiated chords rather than sustained chords, and you can't change the voices used.
One possible solution would be to create a simple style file that doesn't play anything except a sustained chord using a particular voice. I'll see if I can create such a style file and post it here. If you want to use a particular voice for the left-hand part then you would need to use a free utility program to "revoice" the style file, which isn't nearly as difficult as it might sound.