That’s the fun part. In my case, on the battery side of each wire of the JST, I cut off the connector and solder a small piece of metal washer on each wire wherein a magnet can be inserted between the cells.
Or you can reconfigure an 18650 battery holder to have protruding connectors using bolt/nut. I solder a small alligator clip on each wire end on the JST wherein it can ‘bite’ on the protruding bolt, thereby giving physical contact to the Balance connector of your charger.
As I said this is the fun part because the it’s not the usual ‘put in the cradle and wait for the green light’ to come on. Your imagination will be the limiting factor!
Lately I have charged my NCR19650PD cells in parallel, as shown in this post of mine: (the battery-holder has been reconfigured for Parallel charging, originally series-connected) POST #406