I'm not sure if you are asking about a portion of yiour design, or the actual hardware that has been synthesized and the bitstream loaded on to an FPGA.
If you are referring to the final hardware, then yes, you can accomplish what you are asking. At that point it's just hardware and you can design your circuit however you wish. Adding an external D-A chip is completely reasonable.
If you are asking if you can design a D-A converter in MATLAB or Simulink, then the answer is "probably not". HDL Coder emits digital HDL code, bit-true and cycle accurate. Neither VHDL nor Verilog support analog semantics, and HDL Coder does not currently support VHDL-AMS or Verilog-AMS.
