First, the size of sensor DOES NOT affect any type of focusing aspect of any lens you attach. Lenses are the device that focus’ light. Sensors (or film) do not perform any focus operations. This concept applies to subjects including depth of field, minimum focusing distance for macro work, infinity focus and others.
All lenses are designed to do one thing; take incoming light and focus it to a point ON A FLAT PLANE inside the camera body. Lenses made by all manufacturers are designed to focus their light at a very specific distance. This distance, which goes by a couple different “official” names, is measured from the flange of the lens mount to the sensor.
The only thing that is affect by the amount of surface area you are trying to focus light onto (i.e. size of the sensor) is the angle of view of whatever lens you attach. If the limited angle of view experienced by a “crop” sensor camera forces you to change the distance from camera to subject, it is this change in distance that affects the aforementioned aspects of focusing. Not the crop sensor itself.
Your second question could be a little clearer. If you are talking about magnifying the image in a viewfinder, as previously mentioned you can attach a magnifying eyepiece adapter. Or, you could also go with a magnifier that attaches to the LCD screen on the camera back.
If you are talking about seeing more of the overall image than what your particular viewfinder shows – in other words, your model of camera only shows less than 100% viewfinder coverage – then, no adapter on the viewfinder itself will fix this. Depending on your model of camera, the LCD itself (in live view mode) may provide more coverage.