When you create a replica, you are stuck with it. You need to create a whole new replica if you want to add a feature class. Could ESRI please make a tool for managing an existing replica's datasets?
I don't believe there is an out-of-the-box way to accomplish this, but it is certainly possible to implement this in ArcObjects. If you're familiar with ArcObjects you can quickly build a tool to implement this and the bulk of the logic you'd need is shown on the following page.
How to add a feature class to an existing or table to an existing replica
The implementation will vary depending on how you want to implement this. I typically leverage it as a console application or build my own GUIs to run this process outside of ArcGIS Desktop, but you could wrap this into a custom GPFunction pretty quickly.
Please vote up the following idea:
Add a Feature Class to an existing Replica
Retrieving data ...