I'm trying to add detail to this texture like making the lips more realistic but I'm not sure how I can achieve it.
Can I merge another image onto lips or would that mess with the texture.
Edit: I am using Photoshop CC 2018
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Of course you can add textures by merging images. There probably are tutorials, but it's not really difficult to imagine how it could be done.
A fairly simple method:
Erase everything except the lips
Scale and position the texture over the lips
Set the layer blending mode to Overlay
Note: If the lips don't quite match/align, you could use the liquify filter to distort them to fit.
Here's an example showing steps 4 to 5. The result is quite subtle, which is probably key to not making it look too fake.