This is a bit involved, and I do not have time to do custom code.
This code adds path/file name to a list box from a worksheet. You will probable want to do it using the Dir function (see VB Editor help).
Private Sub UserForm_Initialize()
Set DataSheet = ThisWorkbook.Worksheets("data")
Set MyList = DataSheet.Range("FileList")
Rw = 1
FileListBox.Clear
While MyList.Cells(Rw, 1).Value <> ""
FileListBox.AddItem
FileListBox.List(Rw - 1, 0) = MyList.Cells(Rw, 1).Value
FileListBox.List(Rw - 1, 1) = MyList.Cells(Rw, 2).Value
Rw = Rw + 1
Wend
End Sub