1- Is it possible to edit the source code of a package (for example stereo_image_proc) and recompile it to be used in an other project?

stereo_image_proc is BSD-licensed, so you are welcome to reuse the source code. All packages have the license listed in their manifest.

2- For stereo_image_proc, could I specify the paths of two images (from my hard disk) and make them inputs for the stereo_image_proc in order to calibrate, rectify and extract disparity map from them?

Jeff's suggestion will work. It's probably simpler to use the stereo_image_proc API, the only caveat being that this API is not considered public and may change in future releases. You would load the raw image pair as in the PublishingImages tutorial, and load CameraInfo's with your stereo calibration (see camera_calibration_parsers). Then fill a StereoCameraModel with the calibration, and pass everything to StereoProcessor::process() - you can look at the stereo_image_proc code for those steps.

Note that stereo_image_proc will create rectified and disparity images, but the cameras must already be calibrated. The camera_calibration package can help with that.

Patrick