I don't think the footprint editor will let you add an actual "via". But you CAN add "pad(s)", and define them so they look like vias, and work like vias. It always seemed to me that a via was nothing more than a thru-hole pad, but it didn't have a component soldered to it.
Study the thread that @tdarlic referenced in his post. Some Very Wise people expended considerable thought and effort to make that thread a complete and accurate answer to your question. They are deserving of fame and fortune. Bestow praise, honor, and gifts on them in exchange for the valuable knowledge they have imparted to you.
Hand soldering will be difficult no matter how you go about making this footprint. Think about it:
- The purpose of that big copper island in the middle of the footprint is to conduct heat AWAY from the chip.
- The vias, pads, thermal conduits, or whatever you want to call the things, are there to improve that thermal transfer efficiency.
- To form a solder joint with something touching that copper island, you must force more heat INTO the island than can be conducted away from the island, until the added heat raises the temperature to the melting point of solder.
Do you see the fundamental contradiction here? The whole purpose of that copper island is to do the exact opposite of what the soldering process is trying to do. Forget about using hand-soldering techniques to create an effective solder joint with the large thermal pad. Invest in a reflow oven, hot-plate, or second-hand electric skillet that can uniformly heat the whole board at once (as well as all the components on the board) to soldering temperature.