To make the pigtail, you will need a leak-proof hose. Chose to use a long double-bard union hose. This will now allow you to connect the two different kinds of tanks or gas cylinder bottles. Then, cut the pigtail in half and join the NA POL (the hose segment with covered POL fitting in).
NOTE: You need to follow the provided step by step instructions for doing a tank-to-tank refill, following all safety precautions, as this is known to be an illegal refuelling method. This is a slow and somewhat inefficient process.
My choice was to join the other half of that pigtail you cut in half (the one with the invented POL fitting at the end) to the same regulator fitting + hose, and you will be able to have a gas-tight connection between a gas cylinder and your regulator. Go ahead and use your custom tank-to-tank pigtail. The one connecting your gas cylinder valve to the POL installed regulator.
When you build a custom pigtail (self-made), make sure you diligently perform a soapy test each time you work on the propane system. If you are tempted to install a 12.5kg regulator in conjunction with using a 3kg diaphragm type valve, with hose and adapter, keep in mind the regulator well not fix in the cylinder bottle valve. You might want to check with your gas cylinder bottle whether that will work with the connection.
To do a tank-to-tank refill from a 3kg cylinder bottle, you will need to construct another custom pigtail like the one described for the diaphragm valve type, with a POL fitting connector at one end joined to the POL + hose.