KiXtart is very loosely structured, so you can break the code over as many lines as you like - indeed it is encouraged where it makes the code easier to read.

You don't need a special character, but you must break the code in a sensible place. Don't break in the middle of a variable name for example.

Examples:
Code:

Function udfMyFun(

   $Var1,

   $Var2,

   $Var3

   )

Code:
   $CSVString=

      '"'+$Var1+'",'

      + '"'+$Var2+'",'

      + '"'+$Var3+'",'

      + '"'+$Var4+'"'