In short, yes. The long answer is it all depends on the ability of the hacker... coder. This would require lots of time and extensive programing knowledge.
SMP simply relays information to your client. Even if it relays the same messages as it does in the SSP mods (such as spawn a shark in MoMonsters), all users on the server would need to have downloaded the mods themself otherwise it would be sending unrecognized messages to a normal client. Simply, theres only so much the server can do. SSP mods are awesome because they literally edit your game. Servers cannot do that.
