is it possible to add new connections to a fitler port via the pipeline browser

ParaView Support
7

You are missing the following xml code : multiple_input="1"

It may look like this :

      <InputProperty clean_command="RemoveAllInputs"
                     command="AddInputConnection"
                     multiple_input="1"
                     name="Input">
        <ProxyGroupDomain name="groups">
          <Group name="sources" />
          <Group name="filters" />
        </ProxyGroupDomain>
        <DataTypeDomain composite_data_supported="0"
                        name="input_type">
          <DataType value="vtkDataSet" />
        </DataTypeDomain>
        <Documentation>This property specifies the datasets to be merged into a
        single dataset by the Append Datasets filter.</Documentation>
      </InputProperty>