Is it possible to take an app that you have downloaded, or one that is on your phone but not on your computer and upload it to eclipse? So basically what i'm asking is can you take an app from your phone and upload it to eclipse?
Are you asking if you can 'open' an application in eclipse (from your phone) to edit it? Typically applications are not distributed with any form of source code, so I'd say the answer to your question is 'no'.
The files located on your phone will be compiled into .class files and packaged up. I suppose technically you could decompile the class files, but you would end up with something pretty messy and unusable.
If you are only interested in running some code from the application it may be possible to extract some of the .class files and use them?