Turning a regular backpack into a camera bag

So I've wanted to get a bag for my camera, lenses and other gear for quite some time. Looking for bags that can take my DSLR, a few lenses and some accessories has shown that it can be quite expensive. Then I thought, I already have a refular backpack I really like, an Eastpak Padded Pak'r, couldn't I use it? I've thought that I could buy pouches for lenses and a smaller bag for the camera itself to put in the bag, but it doesn't seem ideal. So do you guys have any tips in turning a regular backpack into a good camera bag (with everything safe)? I would prefer, almost demand, that it would be modular (so it can be used as a regular backpack too). If not, do you know any cheap, high quality camera bags that can hold up to 3 small-medium lenses, a DSLR and some accessories? Sorry for the layout, wrote this on my phone and paragraphs don't seem to work.