There's a Multiply Accumulate block in the arithmetic library. You could wire the b input to a constant 1 so that it just does simple accumulation, but that might use up a DSP. It's possible that if you set "Use Dedicated Multiplier Circuitry" to NO, that Quartus might optimise away the multiply. You'd really have to see for yourself if that works.The other option would be to add a Pipelined adder, set its latency to one and feed its output into one of the inputs. Again this block is in the artithmetic library.