The NIH has developed a citation style for RefWorks that includes PMCID numbers. If this style is not already in your favorites, it is easy to add and use.

In your RefWorks account, go to the Output Style Manager (located in the menu box on the right side of the page), and search for "NIH." Highlight the result that appears ("NIH - National Institutes for Health"), and click the top green arrow to add it to your Favorites. You should now have the option to use this style when creating a bibliography.