PPP is contained entirely within a single eCos package. So to include PPP in a configuration all you need to do is add that package.
In the GUI configuration tool use the Build->Packages menu item, find the "PPP Support" package in the left-hand pane and use the Add button to add it to the list of packages in use in the right-hand pane.
In the command-line tool ecosconfig, you can use the following command during the configuration phase to add the PPP package:
$ ecosconfig add ppp
In addition to the PPP package you will also need to have the "Network" package and the "Serial Device Drivers" package in the configuration. The dependencies and requirements of the networking package are such that it is strongly recommended that you start with the net template.
See the eCos User Guide for full details on how to configure and build eCos.