Yes, you can do it, if you're the server (host player). Login with your admin password, and type (in console):
\rcon devmap <mapname>
You load the map with cheats (there's no "helpusobi 1" usage for multiplayer).
The map will say "cheats enabled" and so ANYONE can use cheats to spawn enemies, turn on god mode and so forth (and crash the server if you do too much crazy stuff).
Spawning NPC's is easy.
\npc spawn <name of character>
Then move aside (with room for the creature to appear) and it will! They kinda suck though, because in MP they get confused and don't know which player to attack.
Of course you can switch to SP maps in MP. All this is doable, but of course the scripting will be all messed up and you probably won't be able to finish it.
I recommend OJP mod (Basic and Enhanced) and you can try "COop" which makes Single player in Multiplayer a little more playable (Tons of bugs still though).
But to cheat in MP no mods are required. Just load your favorite map with "devmap" instead of "map" command. That's all it takes. Then just type in the cheats like normal. The only cheat that I've tested that won't work is "undying" (the one that gives you 999 life).