Nu har jeg vist fundet ud af det, man kan gøre det på to måder:
Ombyt punktum med komma:
Option Explicit
Private Sub CommandButton1_Click()
Dim rg As Range, r As Integer, s As Integer, rownr As Integer, xcell As Range
Set rg = Selection
For r = rg.Rows.Count To 1 Step -1
For s = 1 To rg.Columns.Count
rg.Cells(r, s) = Replace(rg.Cells(r, s), ".", ",")
Exit For
Next s
Next r
End Sub
Eller bare den simpleste måde, at konvertere til tal!

..fordi excel læste "tallene" som text:
Private Sub CommandButton2_Click()
Dim xcell As Range, rg2 As Range
Set rg2 = Selection
For Each xcell In Selection
xcell.Value = CDec(xcell.Value)
Next xcell
End Sub