First suggestion is to upgrade to 7.17. There have been numerous bug fixes, upgrades, and other improvements to mIRC since 6.21
Secondly, in order to do a script like that you would need to use the ON INPUT event to capture the text you enter, then "convert" it using $encode, then send the message to the person or channel.
Next the person that is wanting/needing to read the message will need to have a script that uses $decode to "convert" the information back to English. By default, $decode is disabled, so the receiver would have to enable it manually.
One problem with giving you a code here, is the fact that anyone else that looks at these forums could take the same code, thus making the encryption useless.
Personally, I think that the amount of work required to do a script (actually two would be needed, one to encode, one to decode) in relation to the security level makes it not worth-while.
I'll post a simple example after I've written and tested it.