I suppose you are already using a kind of theme that halts default text and tampers with it!
There will be an echo command there that echoes the text to the channel window. You could add a -i switch to that echo command depending on how the nick appears. For example if you have the common appearance:
<nick> text text text
then you should use something like echo -i $+ $calc( $len($nick) + 3 )
(plus any other echo switches)
+3 goes there for the < and > signs (2) plus a space after the >
