You need to use 2 of your 8 mix outs (aux, postfade) as stereo fx send and 2 of your 16 input channels as stereo fx return, if you use one external effect or several as a chain. If you want to use effect processors independent and separate, you need to sacrifice more output and input channels. Try to prevent more AD/DA conversions in your fx chain, because this will add too much latency to the signal flow.
The RM mixers are not built with external processing in mind. The SL AI mixers are more flexible in this respect. If you are not using the RM mixer in a network, FireWire gives you access to the processing of DAWs. There are templates in Studio One available, for example, which makes the DAW to your "effect rack".