Per avere un controllo granulare sulle proprietà dei campi non puoi usare comandi SQL, ma puoi seguire l'approccio programmatico.
La guida MSDN cita testualmente :
Using SQL Data Definition Language statements, you can only specify the field names and data types. Use programmatic DAO access to specify all properties.
Ti riporto anche l'esempio tratto da MSDN :
codice:
Dim dbs As Database
Dim tdf As TableDef
Dim fldID As Field
Dim fldName As Field
Dim fldResponse As Field
Dim fldClass As Field
Set dbs = OpenDatabase("C:\MYTEST.MDB")

Set tdf = dbs.CreateTableDef("Marketing Survey")
Set FldID = tdf.CreateField("ID", dbInteger)
    fldID.Required = True

Set fldName = tdf.CreateField("Name", dbText)
    fldName.Required = True
    fldName.Size = 40
    fldName.AllowZeroLength = True
    fldName.DefaultValue = "Unknown"

Set fldResponse = tdf.CreateField("Response", dbMemo)

Set fldClass = tdf.CreateField("Class", dbText, 10
    fldClass.Required = True
    fldClass.ValidationRule = "in('A','B','X')"
    fldClass.ValidationText = "Enter of of A, B, or X"

tdf.Fields.AppendFldID
tdf.Fields.Append fldName
tdf.Fields.Append fldResponse
tdf.Fields.Append fldClass
dbs.TableDefs.Append tdf
dbs.Close
La proprietà che ti interessa è "AllowZeroLength = True"

Ciao.