You are able to use excessive amounts of Haki. Proof being Luffy's G4, with both Rayleigh and Law mentioning how Luffy us using too much Haki. The problem with Enma was, that it forced Zoro to do so against his will. What made it hard for Zoro to control Enma was to constantly try and restrain his Haki. King of Hell is basically Zoro allowing all of that Haki out, which he can do with another blade too.
Anyone that has the level of Haki and will power Zoro has, meaning not many. Most would turn into a corpse wielding Enma.
Is it a special ability enma allows zoro to do or is it something zoro/anyone could do without it if they don't regulate their haki?