You'll need a Latitude or Precision model that has an Expresscard socket -- you can then add an Expresscard with the RS232C port on it:

http://www.newegg.com/Product/Product.aspx?Item=N82E16839158050&nm_mc=KNC-GoogleAdwords&cm_mmc=KNC-GoogleAdwords-_-pla-_-Laptop+Add-on+Cards-_-N82E16839158050&gclid=CPj31tvSyrkCFTFo7AodVGUAEw

The E5530 is one model that has the Expresscard socket:

http://www.dell.com/us/business/p/latitude-e5530/pd

NONE of the consumer-grade Inspiron/XPS models have the expansion socket.