Like this

myVar = math.eval("externalDocumentInMyLib\\thePublicVariableYouWant")

More advanced use :
https://github.com/adriweb/EEPro-for-Nspire/blob/master/EEPro.big.lua#L1519

But there, I'm actually calling a function and taking its return value

You can use any type you want as long as it's lua compatible (through the math.eval)
It has to be a math variable inside a normal .tns document you created as usual.

If you want to do like Make3D - that is, loading the entire source code from MyLib - you can use loadstring(var.restore("my_file\\my_var"))()
In my_var of the TI-Nspire document my_file.tns, you have a string (be careful on its size tho, there is a limit, like 16K chars or so) contaning your source code.

For example :
In my_file.tns (normal document, inside MyLib - don't forget to refresh libraries)
my_var := "function on.pain(gc) gc:drawString('Hello world', 0, 0, 'top') end"
In my_example.tns (Lua document)
function on.restore()
   _, err = pcall(function() loadstring(var.recall("my_file\\my_var"))() end)
  print err
end
This should display "Hello world" (haven't tested yet but you should understand the logic)

Do you know if i can use ".txt" or ".lua" files too?

No, you can't read file, basically. This is why such "hacks" have been introduced.