// Add <legacyDrawing r:id="rId1"/>
VMLPart vmlPart = new VMLPart();
vmlPart.setContents(getVml(0,0)); // corresponds to A1
// you'll need extra VML for each comment
Relationship rel = sheet.addTargetPart(vmlPart);
CTLegacyDrawing legacyDrawing = Context.getsmlObjectFactory().createCTLegacyDrawing();
legacyDrawing.setId(rel.getId());
sheet.getContents().setLegacyDrawing(legacyDrawing);
}