The wizard for the Custom XPath Function (CFX) allows you to specify only one class for CXF. That does not mean that you have to create a "plug-in" project for each of the custom function classes. You can use the wizard to create different plug-in projects for each of your CXF groups. Then later merge these plug-in projects into a single project. See the steps below. 

1). Copy the class files under a single package under the plug-in. Modify all classes under the same package.

2). Merge the plugin.xml file that has extensions for CXF. See the attached file (Filename: plugin.xml) for an example.

3). Under the OSGI-INF folder, you will find an XML file. Copy the XML files from other CXF projects. The name must be unique.

?4). Open the build.properties and ensure that OSGI-INF folder is selected.

Also attached are two Studio archives (Filenames: customxpathplugin_seperate.zip and customxpathplugin_merged.zip) for the application using xpath functions from two separate and then merged plugin projects. Screenshots attached (.png files) highlight the key changes to be done for merging.