Use code to Add Column to Table, How to make nullable

Whenever I have to alter existing tables in the MSJet (4.0) database of a VB6.0 app that is already in production, I do it using code such as:

Dim tbl
Set tbl = cat.Tables(passedTable)
'
With tbl
            .Columns.Append passedField, adVarWChar, passedLength  
End With

passedTable, passedField and passedLength are set to the appropriate values prior to this.

This way it is transparent to the user.  Sometimes I have to initialize the new field to a value (which I do using code) but sometimes the field can be left blank.  When I leave the fiield blank I run into problems when I create a new record without specifically putting something into the new field.  It is the 'Nulls not allowed" error.

Setting the 'allow nulls' property to yes at the time I append the field to the table would eliminate this problem but I don't know how to do that when adding a field to an existing table.  How do I set the 'allow nulls' (adNullable) property when adding a field to a table?

I do it when creating a new table with the following code:

Dim tbl
Set tbl = New ADOX.Table

With tbl
    .Name = TableToCheck
    Set .ParentCatalog = cat
    ' Create fields and append them to the new Table object.
    .Columns.Append "RecType", adVarWChar, 2
    .Columns.Append "DetectOp", adVarWChar, 2
    .Columns.Append "Base", adDouble
    .Columns.Append "UpgradeVal", adDouble
    .Columns.Append "Upgrade", adDouble
    ' set field Properties
    .Columns("RecType").Attributes = adColNullable
    .Columns("DetectOp").Attributes = adColNullable
    .Columns("Base").Attributes = adColNullable
    .Columns("UpgradeVal").Attributes = adColNullable
    .Columns("Upgrade").Attributes = adColNullable
End With

cat.Tables.Append tbl