Strings are immutable so you can't insert characters into an existing string. You have to create a new string. You can use string concatenation to do what you want:
yourstring = "L" + yourstring + "LL"
Note that you can also create a string with n
Ls by using multiplication:
m = 1 n = 2 yourstring = ("L" * m) + yourstring + ("L" * n)