Step 1: Make sure you expanded the map to the North, West, South or east at least 8 times minimum or even if it's at the max of 10, that's great!

Step 2: Go to %appdata%/locallow/pedepe gbr/City Bus Manager/ Saves and find your save file (example.save).
Make a backup of it, just in case.

Step 3: You need to download HxD, you could try the Hex editor of notepad++ but idk how that one works.

Step 4: Load the save file into HxD

Step 5: Press CTRL + F (or go to search > find > text-string) and type in "mapexpansion" and press "Search all"

Step 6: You will see
that it highlights "mapexpansionnorth", the following steps you will need to repeat with "mapexpansionsouth", "mapexpansionwest", and "mapexpansioneast".
So what you need to focus on is "...mapexpansionnorth....", you click on the first "." after the "h" in "mapexpansionorth", so basically the first dot AFTER the word.
what you will notice is that on the right side at "data inspector" you will see "lnt8" which will have a number ranging from 1-10.
This number indicates how many map expansions you can still do.
So if you have reached the maximum of "10" in your save file, it should say "10".
Simply change the number from 10 back to 1, it should automatically change the numbers on "ulnt8, lnt16 etc etc".
Now repeat this process with the other mapexpansions. After all are reset back to 1, save and load the game and you will see that you have 9 extra expansions!

You can do this infinitely, however depending on your pc, it could lag.
I've managed to cover the whole country of the Netherlands and a part of Belgium as well.

Have fun!
Save often and make sure to always have a save backup incase you accidentally do something wrong while editing the save file

UPDATE: According to @d2_god1 "For latest update, you just need to update the west one"