Firstly, there aren't any copyright or ownership issues providing you don't try to pass these missions off as your own or try to make money out of them. BIS are happy for people to do stuff like this.
Secondly, nominesine is spot on. Use Amalfi's pbo decryptor 1.5 to unpbo the missions - its very easy to use. You can grab it from the Tools section of the Editors Depot.
You will probably find, quite quickly, that you want to do a little more playing around in the Mission Editor, which is great. I wrote a Guide to mission editing for beginners, in Tutorials - Getting started, which you might find useful.