maybe the headline isn't that explicit so here's a better description:
I've layouted a book without any bleed and sent it to the printing firm.
They said, that there are maybe white lines.
So my idea is now to make bleed, which use the data of the last pixel from the site and stretches it as far as the bleed goes.
Is there any way to realize this?
I think the answer is "no," but I'm still not clear waht your pages look like. Can you show us a screen shot?
Content aware scale (in Photoshop) I think will do what you want, which is to increase the size of the graphic by adding pixels. Alternatively just stretch the pic to add bleed.