Example:

db.agg_test.insertOne({"a": [31415, 9265]});
db.agg_test.aggregate([{$project: {"a.b": 1, "c": {$const: "test"}}}, {$addFields: {"a.d": "zzz"}}]); 

Result would be:

 "a" : [ { "d" : "zzz" }, { "d" : "zzz" } ], "c" : "test" }

Projection {"a.b": 1} will remove all scalar elements from the array "a" as per projection definition.

Additional $const expression is needed to use default projection implementation instead of a simplified one.

When projection iterates through array, it replaces deleted scalar values with "missing " values and then $addFields stage replaces "missing" values with objects.