You should be able to declare your own product
(using the org.eclipse.core.runtime.products
extension point) specifying the existing application
id.
So, for example, to can declare your product like this:
<plugin>
<extension
id="product"
point="org.eclipse.core.runtime.products">
<product
name="My product name"
application="org.eclipse.e4.ui.workbench.swt.E4Application">
.... more ....
This example is using the existing application id for the Eclipse E4 application declared in an existing plugin using the org.eclipse.core.runtime.applications
extension point.
If the application you are extending is based on Eclipse 4 your product definition will need to include a property for the application XMI file:
<property
name="applicationXMI"
value="plugin id/Application.e4xmi">
</property>
You will need to look at the existing product definition to find the location of the XMI file.
You may also need properties for the CSS file, theme and resources.