This section has some info on how iNat handles species complexes: https://www.inaturalist.org/pages/curator+guide#nodes
I have no knowledge of Polygonum, so I can’t comment there. However, I think general procedure would be to make a flag on the taxon/taxa and initiate a discussion with curators there while including your reasoning and resources. If it seems like adding a complex is a good choice, then curators may be able to add it.
There are also lots of other forum posts on species complexes that may be worth checking out:
https://forum.inaturalist.org/t/insect-species-complexes-to-create-in-inat-or-not-to-create/15206 for example