Call Oracle Function As Part Of Parametrized Query?

I have a number of automatically generated data access classes to handle things like inserts & updates. These look like:

cmd.CommandType = CommandType.Text
cmd.CommandText = "UPDATE myTable set f1 = :f1, f2 = :f2, f3 = :f3 where id = :id"
cmd.Parameters.AddWithValue(":f1", _f1)
cmd.Parameters.AddWithValue(":f2", _f2)
cmd.Parameters.AddWithValue(":f3", _f3)
cmd.ExecuteNonQuery()

If I were to re-write the sql to do what I want, it would look something like

cmd.CommandText = "UPDATE myTable set f1 = pkg.getMyValue, f2 = :f2, f3 = :f3 where id = :id"

Is there any way to do this by setting _f1 to a "special" value, without changing the update SQL or the way the parameters are set? Only in certain cases will I need to set the value of f1 - most of the time, it will be null.

Using .net 2.0, and system.data.oracleclient against oracle 11.