In short, yes. You can install whatever you like on distributions that use apt-get (Raspbian, RaspBMC), including packages for desktop environment, using
sudo apt-get install <package name>. That being said it tends to be easier to start with an image that matches most of your requirements (to save searching for packages). With XBMC you can also install 'addons' for a variety of purposes through the XBMC GUI (including BitTorrent with Transmission).
If you would like a media center that shares files out of the box I would recommend OpenELEC which can be easily installed with BerryBoot. However, OpenELEC does not use a package manager such as apt-get so you will need to install anything additional manually or as an XBMC addon.
Using the OpenELEC image as default boot with BerryBoot will automatically set up a network share on loading that you can use to copy additional images to your SD card over a windows network by entering
\\<Your Pi's IP Address>\Media\berryboot\images\ into the explorer address bar.
Not sure what this translates to on a mac but if you find out please let me know.
I've also written a QuickStart Guides for RaspBerryPi and OpenELEC if they're of any help - feedback welcome.