You can do that simply using the function cat().
If you want to use a list composed by @var1 and @var2 in your Fill & Clean task, you have to write in your parametric option the following line:
cat(@var1,@var2)
Generally speaking, cat() function allows you to concatenate 2 lists only.
If you want to concatenate three or more (e.g. @var1, @var2 and [“A”,“B”,“C”]) you have to write:
cat(cat(@var1,@var2),[“A”,“B”,“C”])