How can i read/write data from/to a PN/PN Coupler in CFC after it's correctly configured in HW Config.

Is there any standard function block in APL for that purpose?  

There are several methods depends on your use case and/or expertise.

1. You can follow this article How to integrate the PN/PN coupler or the PN/MF coupler into PCS 7?  and purchase the associated licensed library solution from Siemens.  (9LA1110-6CG20-1AA1)

2. You can create extremely basic I/Q interface with your CFC logic (basically no diagnostics, just raw value communication via PN IO, add your own watchdog, etc.)

3. You can create your own custom blocks to do what the paid library does from #1 and use that.  Sort of a hybrid solution of #1, #2, much more cost effective but still giving most of the benefits.  It does require the highest expertise level though.

Edit:  I just checked and the new updated library has received a much more reasonable price than before, it's probably hard to use #3 solution for the price now unless you already have historical self created blocks.