Since the WLCSP chip already has tiny solder balls, would it be possible to hold the chip in place on the breakout board with tape and then heat it with a heat gun? I'd have to monitor the surface temperature to ensure it doesn't exceed the max reflow tolerance (~260C).
Is it possible? Yes. Keep in mind its harder if you have other parts already on the board (use foil as a heat blocker). And the small size vs the heat gun's diameter and air flow speed. I've done it on a similar wlp chip using a small butane soldering iron with the heat gun option. About the diameter of a AA battery.
But overall its not a good way to do it. And especially not an easy way to do multiple at a time or repeatedly. There are other solutions you could try like a toaster oven converted for soldering or a pan etc etc but if you need to do multiple a proper hot air soldering station is used.
Alternatively, you can solder to it dead bug style. A 6 pin WLP like that is easy (relatively) to solder to with some spare wire as there would be no overlapping. Still not a walk in the park though. http://dangerousprototypes.com/blog/2012/11/15/fine-pitch-bga-deadbug-soldering/