Sure you can. The app does not limit which trains you can use. Just add any missing train manually.

Would be interesting where you’d like to go. Because up to Ankara or Konya the stations do show up. So I think it’s not the direct way to Istanbul you are writing about (the following wouldn’t apply for this). So:

In theory it’s possible (as written before).

In fact it doesn’t make any sense. Ok, nearly.

Mainly because of:

• getting reservations will not be easy (for high speed trains)
• most conventional long distance trains convey 1st class only (ok, no problem with 1st class pass)
• train travel in Turkey simply cost next to nothing at the moment - for instance Ankara to Kars is about 6 € 1st class for 1310 kilometers - and you get a reservation

In TR the network is VERY limited

Due still to covid and many trains are still CANCelled.

You never were able to go anywhere-along the coast are simply no lines, its mainly to go to Ankara.