Noah would usually avoid inscribing an item that had already gone through one round of forging.
The Elemental forging method required specific meanings for each product that he wanted to create, and fusing items that had already been inscribed usually led to a conflict of wills.
However, Noah didn't need to change the will when fusing the new center of power with his heart. He had used the same imbued higher energy to force their tissues to merge.
That was necessary to make his body consider the new organ as part of Noah's existence. He couldn't just place it there because he needed to rely on the resilience of the magical beasts to harmonize that addition.
Yet, the real harmonization would arrive after the next breakthrough of his body. Now he had to deal with the consequences of his new organ on his own.