Thanks a lot for you help I am able to add the attribute using following code:
FDoc := CoDOMDocument30.Create();
FDoc.Async := false;
FSubmission := FDoc.CreateElement('Submission');
FAttr := FDoc.CreateAttribute('xmlns:xsi');
FAttr.Value := 'http://www.w3.org/2001/XMLSchema-instance';
FSubmission.Attributes.SetNamedItem(FAttr);
FAttr := FDoc.CreateAttribute('xsi:noNamespaceSchemaLocation');
FAttr.Value := 'layout-topologie.xsd';
FSubmission.Attributes.SetNamedItem(FAttr);
FDoc.AppendChild(FSubmission);
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