How to get an auto focus motor on a non auto focus Nikon DLSR?

Maybe an add on, where the lens mounts to the camera. Or maybe a way to take the camera apart, and physically add it in yourself.

No. There's no space to fix the motor and gears. Further, the Lens is fixed.

There's a work around - get (or make) a follow-focus rig for your lens. Originally used by movie cameramen to move smoothly and swiftly to and between focussing points, they also became popular with sports and wildlife photographers in the 60s.

You can't add an autofocus motor to your camera, but you can add one of these to any lens, and it might be a reasonable alternative.

Search for "follow-focus" - there are lots of DIY versions on YouTube to make from as little as $1 (I even came across one made of Lego!) or you can buy a commercial version for $150- $1300.

Yes, buy a Nikon D90.