re: between first and second sentences
Remembering my primary school teacher who said something along the lines of A sentence starts with a capital letter and ends with a full stop (a period if you're in the States) then it might work if you look for the first full stop, and replace it with a full stop and the string you want to add. Your example shows a new line too, so I've thrown that in (vblf):
Sub blah()
myStringToAdd = "string"
For Each cll In Range("A1:A100").Cells
cll.Value = Replace(cll.Value, ".", "." & vbLf & myStringToAdd, , 1, vbTextCompare)
Next cll
End Sub
or you could incorporate that new line within the string variable:
Sub blah2()
myStringToAdd = vbLf & "string"
For Each cll In Range("A1:A100").Cells
cll.Value = Replace(cll.Value, ".", "." & myStringToAdd, , 1, vbTextCompare)
Next cll
End Sub