Finally solved my own problem! Just adding here so than anyone else stuck in the same problem can find the answer.
ObjectFactory factory = Context.getWmlObjectFactory();
CTSimpleField ctSimple = factory.createCTSimpleField();
ctSimple.setInstr(" MergeField fieldValue \\* MERGEFORMAT ");
RPr RPr = factory.createRPr();
RPr.setNoProof(new BooleanDefaultTrue());
Text t = factory.createText();
t.setValue("fieldValue");
R run = factory.createR();
run.getRunContent().add(RPr);
run.getRunContent().add(t);
ctSimple.getParagraphContent().add(run);
JAXBElement<CTSimpleField> fldSimple = factory
.createPFldSimple(ctSimple);
// P
P para = factory.createP();
para.getParagraphContent().add(fldSimple);
Body body = factory.createBody();
body.getEGBlockLevelElts().add(para);
wordMLPackage.getMainDocumentPart().addObject(body);