Add element into collection object if not exist

Question

I have Collection of objects like

[{ key1: 1,
   key2: 3,
   internal_oject:{
   1:'some_value1',
   2:'some_value2',
   3: 'value3'
  }
 },
 { key1: 3,
   key2: 5,
   internal_oject:{
   1:'another1',
   3: 'another3'
   }
  }
 ]

Using aggregation I need to add missed internal_oject.X values ( where X - is value is known in advance(i can bind it in aggregation query)). For example, I already know that X = 2 (I know this key can be missed).
How I can add inside internal_oject add 2:'missed' (maybe using $project)

After executing your aggregation query I expect to see something like this: objects like

[{ key1: 1
   key2: 3
     internal_oject:{
       1:'some_value1'
       2:'some_value2'
       3: 'value3'
    }
  },
  { key1: 3
    key2: 5
    internal_oject:{
       1:'another1',
       2: 'missed', //<---- must be inserted
       3: 'another3'
    }
  }
 ]

I find similar issue MongoDB - Projecting a field that doesn't always exist But, I need to add element inside internal object (not root)