As the title says i want to know if it's possible to travel between the slums and the old town after i've completed the Story. Second question for this - is it possible to travel back to slums while the story is in the old town, if yes, is there a fast-travel way available?
Once you have been to both the Slums and Old Town, you can fast travel between them. You do so using posters in the main safe houses (the towers). Below are the photos of the map icons and the posters. You can fast travel between them even after you have beaten the main mission.
Go to either tower/base, slums by the merchant on the 1st floor or old town also by the merchant and click the poster hanging to warp between