You can manage this using customization scripts in TMS Data Modeler.
Here is an example:
procedure OnColumnGenerated(Args: TColumnGeneratedArgs);
var
NewType: string;
begin
if (Args.DBField.DataTypeName = 'Number') and (Args.DBField.Size2 = 0) then
begin
if Args.DBField.Size <= 10 then
NewType := 'Integer'
else
NewType := 'Int64';
if not Args.DBField.Required then
NewType := 'Nullable<' + NewType + '>';
Args.Field.FieldType.BaseType := NewType;
Args.Prop.PropertyType.BaseType := NewType;
end;
end;