Merging CSV data into TopoJSON file

I have a file, Council_Districts.topojson, with the following structure:

{ type: 'Topology', arcs: [...], transform: {...}, objects: {
  Council_Districts_2016: {
    type: 'GeometryCollection',
    geometries: [
      {
        arcs: [...],
        type: '...',
        properties: {
          'DISTRICT': 0,
          'ID': 0,
          'OBJECTID': 0,
        }
      },
      ...
    ]    
  }
}

I also have a `Sample_Data.csv’ file with the following structure:

DISTRICT, varA, varB, ....,
0, .1, .2, ...

I’m looking to combine these two files such that my output file has the following structure:

{ type: 'Topology', arcs: [...], transform: {...}, objects: {
  Council_Districts_2016: {
    type: 'GeometryCollection',
    geometries: [
      {
        arcs: [...],
        type: '...',
        properties: {
          'DISTRICT': 0,
          'ID': 0,
          'OBJECTID': 0,
          'varA': .1,
          'varB': .2,
        }
      },
      ...
    ]
  }    
}

Googling around, I’m finding the previous answer has been to use the external properties flag on the topojson cli, e.g. topojson -e blah, but this flag does not seem to be available within the current topojson 2.0

mapshaper input.topojson -join "input.csv" keys=DISTRICT -o output.geojson