Create a process that do something like this:
PICK ONE OR MORE FROM 'Query name'
ENTER NEW SubsetParam
INSERT YourObject IN SubsetParam.Subset.Members
This assumes that the query shows YourObject. SubsetParam is a temporary object which has a references to the Subset object which it displays as a drop down. Displaying this object lets the user select the subset to insert objects into. It also assumes that the Subset object has a multiple reference to YourObject in its Members attribute