An old changelog for phpPgAdmin says that there is an interface for creating and editing composite types, but I can't find it anywhere. Is there a GUI for this in phpPgAdmin?
If you look in the tree navigation, go to cluster->database->schemas->schemaname->types, which will list all custom types, and give you the option of creating new types (including composite types). If you dont have a "types" section, it's possible it's hidden using the "advanced objects" flag in the config file; make sure that's on.