This is a pretty straightforward task:
Dim text = "Line 1" + vbCrLf +
"Line 2" + vbCrLf +
"Line 3" + vbCrLf +
"Line 4" + vbCrLf +
"Line 5" + vbCrLf
Dim lines = text.Split(New Char() {vbCr, vbLf}, StringSplitOptions.RemoveEmptyEntries)
text = String.Join(vbCrLf + "- ", lines) + vbCrLf