Probably the easiest way to do this would be to tell yarn to use an editor
for your reply messages that has a sufficiently rich macro language to do
what you want, using an editor macro. For example, most of the
enhanced versions of vi (e.g., elvis, vim) can be configured to grab
the text after the From: line and use it to construct a prefix. They
can do this automatically when you open the editor. In elvis, you would
put the macro in its, elvis.arf file. In Vim you would make the macro
an ``autocommand'' within its .vimrc file.
Slightly more complex would be to use an editor that has decent shell
escape commands. They you could run your message through an awk or perl
script to accomplish your task, while in the editor. The same awk or
perl script could be run as a post processor before you actually
transported your replies to your ISP for mailing.
Incidently, I created the prefix above (<ad>) using one search/replace
command in vi. One problem with with using first/last initials is that
you will not always have enough info. in the From: line to know what the
first and last names are.