Remapping F keys in an Xterm (xvt)

OK... I posted a couple messages last week about getting special strings
of characters to output when I press one of the F keys in the console.
For example, if I press F5, then I would get ^[[6~ to output. However,
I'm trying to figure out how to do the same thing with an Xterm. I've
read the manpage on xmodmap, and Googled until I was blue in the face, but
I haven't found how to send the same codes from within X like I'm able to
do from the console.
Any suggestions?
Thanks,
~M