If there is enough information in EEG signals to generate speech, an intervening layer to "convert" EEG signals into ECoG signals is an unnecessary complication.
Unfortunately, there probably isn't; on top of that, EEGs are extremely susceptible to noise from nerve signals going to the head/face, such as clenching your jaw or raising your eyebrows
ECoG has much higher temporal and spatial resolution than EEG. Conversion from EEG to ECoG is not likely, the opposite can be done, but not very useful though.