An array is not an object, and therefore doesn't have any properties or methods you can call. You need to resize the array and then add the item like this
Dim counter as long
vList as variant
vList = RefData.NameList
iCount = UBound(vList)
If RefData.Mode = "On" Then
vlist = Application.transpose(vlist)
counter = ubound(vList)
redim preserve vlist(1 to counter +1)
vlist(counter + 1) = -2
vlist = Application.transpose(vlist)
End If
The transposing is necessary because assigning a range to a Variant always creates a 2D array and you can only resize the last element of an array if you use Preserve to maintain its contents.