Conditional rule in an output style

Is there a way in a style (e.g. NIH) to tell the style to put in one field but if this field is empty to put in another field? I am  trying, in the NIH output style, to have Endnote enter the PMCID if there is one in that field but want the PMID to be added if there isn’t a  PMCID. Thanks for considering my question.