Go read this tutorial at MTS: http://www.modthesims.info/wiki.php?title=Sims_3:Installing_Package_Files
Likely, what's happening is your internet browser is trying to open up the .rar file. When you download, right click and select "Save" instead. In order to extract the files you'll need to download WinRar (free).
Download Delphy's CTU from MTS. Use that program to open the tattoo/whatever package you want to alter. Click the "Part Category" tab and look at the field called "CAS Part Type." Check the box for the Accessory type you'd like. Hit Commit. Save.
In general, no. However, I'm open to new ideas, and if you have an idea that I really like, then I might end up making it.