Here is something I came up with. You'll have to adjust the code to work with your existing macro as well as have it dump the random numbers into the column of your choice (I would set the start cell in the beginning of the macro)
<font face=Courier New><SPAN style="color:#00007F">Sub</SPAN> Test()
<SPAN style="color:#00007F">Dim</SPAN> Start <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">Integer</SPAN>
<SPAN style="color:#00007F">If</SPAN> ActiveCell.Row = 1 <SPAN style="color:#00007F">Then</SPAN>
ActiveCell.Offset(1, 0).Select
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
Start = ActiveCell.Row
<SPAN style="color:#00007F">Do</SPAN> <SPAN style="color:#00007F">Until</SPAN> ActiveCell.Row - Start = 5
ReRun:
<SPAN style="color:#00007F">If</SPAN> ActiveCell.Value = "" <SPAN style="color:#00007F">Then</SPAN>
ActiveCell.Value = ActiveCell.Offset(-1).Value + 1
<SPAN style="color:#00007F">ElseIf</SPAN> <SPAN style="color:#00007F">Not</SPAN> ActiveCell.Value = "" <SPAN style="color:#00007F">Then</SPAN>
ActiveCell.Offset(1, 0).Select
<SPAN style="color:#00007F">GoTo</SPAN> ReRun
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
<SPAN style="color:#00007F">Loop</SPAN>
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN></FONT>
I'm sure someone else will come up with a more efficient way to do the same thing... this is to get you started.