from the rules: "Unless specifically announced before the competition, you must use only one virtual synthesizer for all sounds including percussion."
So, if a sound is generated by another plugin it clearly is not allowed.
(Note: In my very first OSC I made a record crackling sound at the start - using Dune. The sound was made using just the synth for both sound and modulations. I expect the same could be achieved with Serum and many other synths.)