Need help with editing strings in a stringList

I have to open a text file read in each line into a TStringList.
Then I have to make changes to each string in the TListdtring
and write the chnaged strings to a list box.

Sample of the sting:
PART,1572315-111B,,,1,,1572315-111B,6

What I have to changes:
After the 1st comma I must insert ss1
After the 2nd comma I must insert ss2
After the 3rd comma I must insert ss3
and the 2nd last string must be 1572315-111B.cdl

The final string must look like this:
PART,1572315-111B,ss1,ss2,1,ss3,1572315-111B.cdl,6

I got this far:-

var
MyString : TStringList;
i : integer;
begin

  Mystring := TStringList.Create;
  Mystring.LoadFromFile('C:\Test.txt);

  for i := 0 to Mystring.Count-1 do
        listbox2.Items.Assign(Mystring);