How to concat a buffer

Hi Andrei, sorry for my delay. I have traveles last week!
thanks a lot for your help and advices.
In response for your answers, I using Zigate to building a home automation gateway using node-red.
I'll using this flow to salve special variables necessaries to implement a Zigbee commisioning procedure and to do it is necessary to get the nwk short address from specific Zigbee message and send it after.

We can sharing experiences about it!