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+'"'
