Does anyone have a way to modify Ticket to Ride to accommodate six people?
The obvious approach is to simply reduce the number of carriages each player has available, by enough to allow another player to join in. Normally each player has 45 carriages. In the 5 player game that makes 225 carriages available to place on the board. Divide by 6 instead, and you get 37 carriages per person.

So your only issue then is where to get your extra carriages. You can either make substitutions yourself, or buy one of the other versions (e.g. Ticket to Ride: Nordic Countries, or Ticket to Ride: Märklin Edition), which come with other train colours.

I wouldn't expect to see any other significant balance issues with the extra player - Ticket to Ride is a pretty straightforward game.