Yes, you can add another stick of ram within the specs DDR3 1066/1333
It is recommended to get a set of RAM for the system when upgrading memory, this means to purchase a package of 2 sticks of RAM with the same model and size, this is to make sure they are going to work well together.
Still, you can add a second stick of memory (DDR3 - 1600 4GB) and it should work fine.
One thing to consider while installing it, check the dual channel memory configuration guidelines.
This board has 4 DIMM slots, a dual channel memory configuration will use DIMM 1 and DIMM 2 populated.
Here a screenshot of the configuration you should use:
If any additional support is required, feel free to let me know :)
I found DDR3 - 1333 4GB Ram stick .
By the way my new ram stick does not support (PASR) !. Is that an important feature/must have fearure ?
This seems to be a low power feature added in some mobile memory (as per https://www.micron.com/~/media/documents/products/technical-note/dram/e0597e10.pdf)
I believe this is not required for the system to work fine
This is one of those Micron-would-like-you-to-think-it-is-a-must-have-feature things...