variable length shift register needed

I have never used DSP Builder but what you described is often called a barrelshifter. Perhaps it is called that in DSP Builder.

In verilog you can create a barrelshift with code like this:

assign c = b << a; // right shift by variable length

assign f = e >> d; // left shift by variable length

The code above will be very slow since it's combinational. You can either pipeline the operation or use a multiplier block to do the same thing.