Assuming Variable = a string and a global
+ One more global variable containing the basestriing ... lets call it 'Base'
Condition > System > for ... name "index" ... start any ... end any
Action > System > Set Value .. Variable=Variable .. Value = Base&str(loopindex("index"))
str() converts a number to a string
& is the operator to combine two strings
loopindex(name) = the loopindex of the 'for' as an expression
I probaly understood the question wrong, i dont see any use in setting a variable this way in a loop. A whole loop happens inside 1 tick.
I you want to add 'x' to variable every tick. Then you just use the tick loop.
In this case you need a globale 'counter' (wich is a number)
Condition > empty or > System > Every tick
Action > System > add to > Variable = counter .. value = x
Action > System > Set Value .. Variable = Variable .. Value = Base&str(counter)