i agree you are on the right track. the cRIO with digital can simulate an encoder with a velocity value. the velocity can change instantly when coded in FPGA mode.
the result will be a 0-5 or 0-24 style signal. if this needs to be converted to +-10V, external hardware would be required.
if your encoder signal is slow, you could use something like the NI 9263 analog output module to produce quadrature output as well. just beware of the slew rates (100 kHz module). how many edges per second do you need to simulate?