Well, there's at least one way to forward your mail from yahoo without having yahoo-plus, but you need to have a Linux / Unix account running somewhere and install the "fetchyahoo" Perl script (google on "fetchyahoo") and then have the *nix mail program redirect to google.
I actually do this, so I know it works, but I only mention it in the event that you or someone you know works on a platform that could make this happen.
There may be other utilities out there that do the same thing... i.e. the way it works is "fetchyahoo" longs in to your Yahoo mail account every so often and downloads your mail.
See for example "YahooPops"
http://www.ypopsemail.com/
It will take some work setting up and installing. Good luck.