RB850Gx2 Can add external antennas ?
You can. But where do you would like to connect them? This board does not have any radio.
which product can integrated with RB850Gx2 and broadcast wifi radio ( No AP solution ) ?
There is no option to have 850gx2 with any type of radio, because this model simply do not have radio integrated or any available pci slots you could use to add such radio cards.
Note down the requirements for the device and select according to them some other device. If you are not sure, just write what you have selected and what functionality do you expect. We will give you some advice then.