Um, off hand I think you would have to use some DHTML to do this and not necessarily use AJAX. I never tried doing that before with JSF components so I don't know how that would work since the the DHTML is primarily all client side and JSF requires the backing beans and in Seam's case EJB.

You could do this easily with Ajax4JSF if you wanted to (if you're using Richfaces then you're already using this). Several advantages over DHTML eg. server-side validation, not trusting the client...etc.

Say you had a Bean called MySerialNumbersBean and it had a List of Serial numbers called mySerialNumbers (with the getters and setters). In your page you would have something (roughly) like this:

<s:div id="serialNumbersDiv">
 <ui:repeat value="#{mySerialNumbersBean.mySerialNumbers}" var="serialNumber">
 <h:inputText id="serialNumber" value="#{serialNumber}">
 <a4j:support event="onblur" action="#{mySerialNumbersBean.mySerialNumbers.addSerialNumber()}" reRender="serialNumbersDiv"/>
 </h:inputText>
 </ui:repeat>
</s:div>