Various things relies on Steam API & I did not studied all of this, best might be to ask to ReHLDS guys but most of them gone & all stagnant. Faking icons is partially possible (like discussed here), but apply other properties to avoid unranked servers on trackers & cie is harder.

Secondly, this is imprecise that you exactly want, if you just wish to set real SteamID just to be visible via 'status/amx_who' (but no point about it), or if you want to apply other properties on servers related to Steamers (as icons on scoreboard & cie).

Firstly, you do not use AMX Mod, but AMX Mod X, so I do not care about providing you help in fact!