Can a sub be hooked up to speaker posts?

This is the arrangement for the REL sub I have, but most people want to use the sub's crossover. In this case, you have to run the amp output to the high level input on the sub, and then out from the sub to your speakers. In the case of amp>speakers>sub, you would only use the sub for very low frequencies, to augment the full-range speakers and use the sub's crossover to limit the upper frequencies from the sub. I need to know more about your system to answer your question.

I also have REL and run it off the speaker outputs, which REL recommends. But as Tgrisham says, we don't know what sub you are using. If it is one that can do it , then listen and see which way you like better. It will largely depend on the quality of your main speakers and the interaction of your system with your room.

So, instead of running the sub from the amp, it would be amp, to speaker to sub.

it's implied in your statement you're using hi level connections for your sub, so yes, this is the same as connecting to the amp, except you have a little section of speaker cable in the middle.