Simply change the variable .clanprefix or .clansuffix on the client object.
Example, changing John's 1st tag to say [SAdmin]:
In console
findClientByName("John").clanPrefix = "[SAdmin]";
Or, in a script, you'd package the GameConnection::autoAdminCheck(%this) and parent it, so that you can make a check to see if they need a special tag.
You can even change the color, or add a picture:
findClientByName("John").clanPrefix = "<color:3388FF>[SAdmin]";
This will make it
[SAdmin]John: Hello, I am John.
Or for an image:
findClientByName("John").clanPrefix = "<bitmap:Add-Ons/Weapon_Sword/CI_Sword>";
(Note that just like all CI icons, Item Icons, and Brick Icons, the client needs to have them, or else they'll see the default icon. A skull for CI icons, and a question mark for item and brick icons.)