Generating ODI Packages

Using the ODI SDK, it is indeed possible to perform almost any tasks you can do in ODI Studio : http://docs.oracle.com/cd/E29542_01/apirefs.1111/e17060/toc.htm.This Java API can also be used in Groovy scripts executed directly from ODI Studio (Tools -> Groovy -> New Script).Groovy is a programming language for the JVM, it uses a syntax similar to the Java Syntax with some shortcuts and it's dynamically compiled. With a few exceptions, Java code can be used in Groovy.

Michael Rainey did a nice presentation to introduce the ODI SDK, Groovy and some use case. Here are the slides : https://s3.amazonaws.com/rmc_docs/biforum2013_slides/odi_mclass_6_sdk_groovy.pdf

I guess this complete example by the same author might be interesting for you as it adds a few columns to the tables of his Foundation layer (equivalent of your layer 1 in the Oracle Information Management Reference Architecture) :http://www.rittmanmead.com/2012/05/oracle-data-integrator-11g-groovy-add-columns-to-a-datastore/

Once you understand the concepts, the Oracle Data Integrator team posted a nice script to automate the creation on their blog a few years back. I think it was tested on 11.1.1.5 so it might need some adapations but this is a nice starting point : https://blogs.oracle.com/dataintegration/entry/interface_builder_accelerator

One limitation with the SDK is that there is nothing to use the versioning capabilities of ODI.

Better to create package, interfaces using GUI instead of SDK. that is easy to implement, debug and time-saving.