Steps to reproduce

Currently, there is a possibility to work with unmapped enums on the direction database -> C# code. But the backward direction is not possible: sometimes it's really worthwhile to have the ability to send the C# enum which doesn't have the corresponding postgres enum with automatic conversion of that C# enum into the underlying integer. The practical example is described here (as the issue of the linq2db project). So, let me just define the test which shows the technical problem.

Suppose, we have a database with the table:

create table person
(
    name     text    not null,
    category integer not null
);

Suppose, we also have a C# enum which logically (but not physically) maps onto the person.category:

public enum PersonCategory
{
    Friends = 1,
    Relatives = 2
}

Consider the following test:

 [Test]
    public async Task FilterWithEnumAsObjectTest_NpgSql()
    {
        var connectionString = "...";

        await using var conn = new NpgsqlConnection(connectionString);
        await conn.OpenAsync();

        await using var command = new NpgsqlCommand("SELECT * FROM person where category = $1", conn);

        var parameter = new NpgsqlParameter("$1", PersonCategory.Friends);

        command.Parameters.Add(parameter);

        await using var reader = await command.ExecuteReaderAsync();
        await reader.ReadAsync();
    }

That test fails with the exception:
System.ArgumentException : A PostgreSQL type with the name 'person_category' was not found in the database

But what would be really worthwhile is to automatically convert enums that don't have the corresponding postgres enums into their underlying integer types prior to sending them to the database as parameters. It's quite a common case because integers are heavily used in postgres databases instead of enums, but in C# code it's good to have a little bit more strongly-typed conventions with enums.
So that feature could help a lot.