Yes, there is a way to concatenate a series of cells using a user-defined function in Excel. Here's how you can do it:

1. Open a new workbook in Excel.
2. Press ALT + F11 to open the Visual Basic Editor.
3. Click on Insert Module.
4. In the new module, paste the following code:
   
   
   Formula:

   Function MULTICONCAT(rng As Range) As String
    Dim cell As Range
    For Each cell In rng
        MULTICONCAT = MULTICONCAT & cell.Value
    Next cell
End Function 

5. Save the module and close the Visual Basic Editor.
6. In your worksheet, enter the formula =MULTICONCAT(A1:A5) in a cell where you want the concatenated result to appear. Replace A1:A5 with the range of cells you want to concatenate.
7. Press Enter to see the concatenated result.
   
   This function will concatenate the values of all the cells in the specified range, regardless of whether they are contiguous or not. You can use this function in other formulas in your workbook, just like any other built-in Excel function.