Is there a way to modify my Shimano shifter so I can trim the front deraillier in flight like in the old days
If you are talking about an STI shifter, the trim function is built-in, it's the half-clicks. If you find that you want to be rid of indexing, which is really unnecessary for an FD, you could always go friction downtube or bar end.
Shimano 9-speed Ultegra and 105 STI shifters have two trim clicks when used on a double and one (pretty useless) trim click when used with a triple.
shimano 7-speed and 8-speed STI's have no trim clicks.
You will have to use downtube, barcons or Campy Ergo front shifters if you want to be able to trim the fd a lot.