If I understand you correctly you have a list of objects that you would like to add to an already existing association.
Depending on the the type of association (1 to 1, 1 to many or many to many) you are able to do this. You also need to do this from the right object.
You need to change the association between the objects, you can do this with the Change Object Action
There are 3 types you can use SET, ADD or Remove. When you have used SET then you drop the existing list and replace it with the new. If you choose ADD, you keep the existing list and add the new list to it. If you choose REMOVE, you remove the members of the list that are in the new list from the existing list.