you can use substring function to get those chars which needed from existing string and than append your char '-' as follows
Declare @ICNumber varchar(20)
Set @ICNumber = 'xxxxxxxxxxx'
Select Substring(@ICNumber , 1,3) -- Return first xxx
Select Substring(@ICNumber , 4,2) -- Return 2nd xx
Select Substring(@ICNumber , 7,4) -- Return 3rd xxxx
Select Substring(@ICNumber , 1,3) + '-' + Substring(@ICNumber , 4,2) + '-' + Substring(@ICNumber , 7,4)