Retrieving extended package object descriptors into Eclipse with Force.com IDE plugin

To retrieve the managed package objects along with your custom metadata, you have to manually add the objects you want to retrieve to your package.xml file. For example, to retrieve a custom object, you might already be retrieving the Account object like this:

<types>
    <members>Account</members>
    <name>CustomObject</name>
</types>

You simply need to add the namespaced object name to this list. This is the "API Name" visible on the object page in the Salesforce UI

<types>
    <members>Account</members>
    <members>packagenamespace__CustomObject__c</members>
    <name>CustomObject</name>
</types>

If you just want to retrieve the fields that you are adding to that object for subsequent deployment, you can specify them in a CustomField type

<types>
    <members>packagenamespace__CustomObject__c.MyField__c</members>
    <name>CustomField</name>
</types>

Remember that the CRUD / FLS rules for any profiles that you are retrieving will include this object / field.

Please also see my response to this similar question about layouts How to retrieve layouts of customs objects belonging to a managed package?.

This technique applies generally to most (all?) objects in the metadata API.