Sorry if I'm not clear. I'm using the original wires from the doorbell point provided by hdb.
Buy the power adaptor..end of adaptor will be 2 wire ends.
Connect the adaptor wires to the doorbell point in your kitchen.
There will be another similar end as above at your outdoor doorbell switch. Connect that end to your nest hello.
What you doing is just to supply power to the nest.
Instead of drilling a hole to connect direct from adaptor to nest hello
Power adaptor >> nest hello
You can skip the drilling by relay the power via the original doorbell wires.
Power adaptor >> original doorbell wires >> nest hello