...I would hold off.

The banned substances list is not meant to be an exhaustive breakdown of every nasty drug or poison. It'd never end if that were the case. I would argue that the list be reserved for substances that agents could be likely to encounter during the course of a mission. That would mean substances that are either:
-- Canonically important or otherwise prone to showing up in a specific canon (FEV in Fallout, Pax in Firefly, or meth in Breaking Bad); or
-- Likely to be used in badfic for one reason or another.
Desomorphine is definitely nasty, but it also doesn't seem to be particularly common knowledge outside of specific countries. I know DemonFiren mentioned that it was part of the game Space Station 13, but said game does not seem to have a great deal of fanfiction associated with it that I could find (in an admittedly very quick search).

So yeah. There isn't a rule. But that doesn't necessarily mean you should add it regardless.

Do you play Space Station 13? Because in some branches (Goon and those adapting Goon-style chemistry) Krokodil exists and does quite precisely that.

In real life things are slightly different, the 'skin-melting' effect mostly comes from impurities and toxic by-products causing necrosis (and worse) when injected.
Of course, desomorphine is still one hell of a nasty drug.