Change carriage returns to %r

Sorry if I this has been answered elsewhere--

Is there a way to automatically edit the text in the input window so that all carriage returns are changed into %r? So that I can for instance copy-paste a multi-paragraph description from a text file and not have MUSHclient read it as me trying to imput several different commands?

Yes, but before we get into that...

What are commands? MUSHclient doesn't send commands. MUSHclient sends lines of data.
Commands are only commands if the server interprets them as directives instead of information.
So then the question becomes, what is special about %r? Is your server going to interpret %r as new line but can't interpret new line as new line?

Anyway, the documentation at http://www.mushclient.com/scripts/doc.php?general=plugin_callbacks has your exact request covered in the example for OnPluginCommandEntered.