We will be adding your currently used user key to the steam version of the game. This will cause the game to get the latest save files for that user key so you can load your latest save.

WARNING DISCLAIMER:
You are working with save files so you are at risk of losing them when something goes wrong.
You are possibly replacing your steam version save files.
I have no idea if anything will change to the save system in future versions of the game.

Go to your game where you want to export the saves from.
There are 2 ways to get your user key depending on where you played this game:

Easiest way would be in the settings tab, in some versions of the game like the main website there will be a user key tab

OR Second way, which should work for all websites, would be from local storage:
Note that I am doing this using chrome. In other browsers the way to get to these settings will differ.
Open the developer tools like below or using ctrl+shift+i
In the developer tools go to the Application tab, then under storage > local storage > https://cityidle.com (link can maybe differ, but will probably contain the name of the game)
look for the user hash value and copy the contents of the value field. This is your user key

Now open the Steam version of the game. We need to get to the developer tools page.
2 ways:
By pressing alt key a toolbar will show where you can go to View > Toggle developer tools
OR you can press ctrl+shift+i

In the developer tools go to the Application tab, then under storage > local storage > https://cityidle.com
Look for the user hash value and replace the contents of the value field with the user key you copied before.

Now if you go to the settings tab you should have your latest saves. So load the one you last saved on and you can now work from this save.

The steam version seems to resync to the old key after a while so it will not stay in sync with the other version.
You could however try to make the other version use the steam key once you successfully moved your game over.
On https://cityidle.com/ you can easily switch the user key via the settings.

Consider liking this guide if it was helpful.