Q:

Java best practices, add to collection before or after object has been modified? 

Say you are adding x number of objects to a collection, and after or before adding them to a collection you are modifying the objects attributes.  When would you add the element to the collection before or after the object has been modified. 
Option A)
public static void addToCollection(List<MyObject> objects) {
    MyObject newObject = new MyObject();
    objects.add(newObject);
    newObject.setMyAttr("ok");
}

Option B)
public static void addToCollection(List<MyObject> objects) {
    MyObject newObject = new MyObject();
    newObject.setMyAttr("ok");
    objects.add(newObject);
}

A:

To be on the safe side, you should modify before adding, unless there is a specific reason you cannot do this, and you know the collection can handle the modification.  The example can reasonably be assumed to be safe, since the general List contract does not depend upon object attributes - but that says nothing about specific implementations, which may have additional behavior that depends upon the object's value. 
TreeSet, and Maps in general do no tolerate modifying objects after they have been inserted, because the structure of the collection is dependent upon the attributes of the object.  For trees, any attributes used by the comparator cannot be changed once the item has been added.  For maps, it's the hashCode that must remain constant. 
So, in general, modify first, and then add.  This becomes even more important with concurrent collections, since adding first can lead to other collection users seeing an object before it been assigned it's final state. 

A:

The example you provided won't have any issues because you're using a List collection which doesn't care about the Object contents. 
If you were using something like TreeMap which internally sorts the contents of the Object keys it stores it could cause the Collection to get into an unexpected state.   Again this depends on if the equals method uses the attribute you're changing to compare. 
The safest way is to modify the object before placing it into the collection.