Try this routing: jnb-mad-ist-bud-ath-hel-fco-jfk

You need to buy 1 extra segment for this, or purchase a One World Visit Europe pass when you buy your DONE4, or purchase one of the segments using a discount airline (although you probably won't get extremely low fares on any of these segments), or take the overnight train between Istanbul and Budapest.

As for St Petersburg, it is an easy 5 hour train ride to/from St Petersburg, see: http://www.seat61.com/Finland.htm#Moving%20on

And don't forget to check out Tallin, a cute UNESCO city that is easily reachable from Helsinki, as well.

Have fun!