Is $convert aggregation supported at any way by the C# driver?

I can't find a way to cast values other than using BsonDocument. 

Example:

var excercice1Aggregate = collection.Aggregate()
    .Match(Builders<User>.Filter.Gte(u => u.Salary, 1500) &
           Builders<User>.Filter.Lte(u => u.Salary, 3000))
    .Group(u => u.Gender,
        ac => new
        {
            gender = ac.Key,
            averageMonthlyExpenses = ac.Average(u => u.MonthlyExpenses), // needs to convert
            total = ac.Sum(u => 1)
        })

User.MonthlyExpense_s is an integer but ac.Average returns decimal which results in strange numbers. So either during grouping or at least on a next step, I need to convert the _averageMonthlyExpenses to an integer.

If I add a new Project stage to convert the averageMonthlyExpenses to Int, using any type of cast the driver throws an exception.

Is there any way to make the cast?