Hi,
Sorry I have not replied for a long time, but I haven’t noticed your post.
I created it in the way, in which you write lists and maps in json format, so list would be like this:
[“item_1”, “item_2”, “item_3”]
And map would be like this:
[ “key_1” : “value_1”, “key_2” : “value_2”]
I have then created even a solution, where you don’t have to write your custom data in JSON, but you can put ID in some of your custom data and then reference it. To illustrate this, let’s say I need to create array of 2 people, so let’s say I have my own class Person, which has ID property, and I have 2 persons with ids “BillGates” and “SatyaNadella”, then I could make a list of them and reference it by this ID by using # symbol.
So I would have: ["#BillGates", “#SatyaNadella”] and this is list of my custom objects I have in different excel, so I don’ need to write them in JSON format.
And I then even solved the problem, that now, those data parses into correct data type, but they are not casted to correct data type. When you go to see that variables, you see them as objects. So I have even created a Powershell script outside of Katalon Studio, which goes through a variables for every test and then creates classes, that cast these objects to the desired data type.
So I then have this line of code at the beggining of a test:
CastedTestData data = CastedTestData.createFrom(this)
And then I am able to write data.SomeVariable inside a test, and that SomeVariable is already in correct data type 
So both of these 2 improvement worked very fine for me, but I haven’t uploaded these yet, because I am not using Katalon Studio anymore.
I found Microsoft Playwright, which is waaaay better Testing Tool than Katalon Studio.
It is new, it is TOTALLY FREE & OPEN SOURCE, still developing rapidly, ported to many programming languages (C#, Java, Python…) but it also have recorder for those, who can’t code. And it is actually much more reliable, faster, better in like everything.
So to anybody who comes to read this post, I absolutely don’t recommend to use my solution anymore, not because it doesn’t elegantly solves this lack of proper types support for test variables in Katalon Studio, but because I absolutely don’t recommend using Katalon Studio to anybody anymore 
Using Microsoft Playwright, you can have much better control of Test Data Variables 
This need for workaround like mine is very huge weakness of Katalon Studio. And it has many other weaknesses I experienced while using it
It is often laggy, whole IDE is based on Eclipse, which is waaay inferior to Visual Studio, which I am using with that Playwright (I use .NET version of Playwright with C#
) It uses Selenium, which is waaay inferior to Playwright.
My opinion is, that all testing tools, which uses Selenium aren’t good, because Selenium isn’t reliable 
Microsoft Playwright is the future of Test Automation for Web, I have no doubts ! 
But It would be awesome, if you could test native apps (iOS, Android or Windows) with Playwright as well, which it is not possible yet. So to test native apps, you still need to use Appium.