I was wandering if it's possible to edit an image with Photoshop after having saved the key frames and before importing the images in lightroom, and if so how to do it. 

I feel more comfortable with Photoshop than lightroom and I'm pretty sure that it offers more features.

Photoshop does destructive editing, Lightroom non-destructive on a XMP Metadata basis. Those are totally different concepts, so you will have to use Lightroom together with LRTimelapse, not Photoshop.