[Discuss-gnuradio] Add multiple gated streams

Anyone have an idea how I can sum (add_ff) multiple, gated streams into a single stream?  For example, the gr-dsd block gates it's output when there is nothing to decode.  I can sum multiple decoders together for a single audio stream, but the add_ff only processes when ALL inputs have a sample.  So what I need is a block that passes all samples, and outputs zeros (or repeats last sample) when input has halted.  Maybe there is a trick to do that with an existing block.

Thanks,
Lou