You are not only the mixer, you are the producer of the mix so there are no rules and no limits to what you can do creatively - though you may not re-track or add any parts.
Sorry for me this doesn't sound you are not allowed to supplement the snare wich was weak and lame anyways and is more of a soundshaping technics than really adding something new. this whole contest is a fraud to me. I am glad i didn't submit my mix there.
They should have exactly listed what is allowed to do and what is not.
When you listen to the rough mix they posted than you can hear that the snare is completely replaced.
I guess they didn't expect that the slightly vague instructions would cause some issues and like they say in their post, in future they'll be more clear about the guidelines.
The only thing that's missing is the "we're sorry for the inconvenience"