ADC undersampling AND oversampling
Since your ADC is already more than fast enough, it would be far better to do all of the filtering, decimating, averaging, etc. that you're talking about in the digital domain, rather than in the analog domain. Then all you need in the analog domain is a low-pass filter that meets the antialiasing requirements of the ADC's actual sample rate.



EDIT: OK, I finally have an idea of what you're trying to accomplish. This is why a diagram is so much better than words. Here's my interpretation of what you're saying:







simulate this circuit &ndash; Schematic created using CircuitLab

The idea is to use dithering and oversampling to increase the effective resolution on each of the analog S/H output samples.

But you can get the same effect in the digital domain this way:







simulate this circuit

This is much simpler, and doing the narrow bandpass filter in the digital domain gives you much better control over its characteristics.

That filter implicitly gives you increased output resolution relative to the raw ADC resolution, because each output sample of the filter is a function of many input samples. In fact, making the bandwidth narrower means that more input samples contribute to each output sample. As long as the input signals aren't synchronous with the ADC clock in some way, they will be "self-dithering" in the sense that the quantization errors (at the ADC sample rate) will be uniformly distributed.

This is the same idea that is used in delta-sigma converters, in which a 1-bit raw ADC resolution is turned into a 24-bit (or more) output resolution by means of digital filtering and decimating.