Yes you can, I did it along with a few others that I know of for sure.
The way I did it was to upgrade my OS at the same time. I feel that makes it easier.
There is a thread that has a ton of modified ALX files for all different devices. It also has links to the OS upgrades.
I simply downloaded the updated OS files, installed them on my PC, then downloaded the modified BlackBerry.alx and Platform.alx files from the thread. Once thats done, all you have to do is replace those two files in the installation directory of the upgraded OS on your PC.
Plug the device in, it should upgrade your OS, and install the PTT modules.
You should call Cingluar prior to the upgrade and have PTT added to your account. Its $4.99/month for 500 PTT mins, and $9.99/month for unlimited PTT.
After the upgrade, you should see the PTT app in your list. The first time you run it, it should initialize. Once thats done, you just add PTT contacts by mobile number. If they have a PTT enabled phone, it will add them, if they do not, it will error out.
Once you get it working, let me know and I can help you test it.
**EDIT: Here is the link to the thread with the modifed ALX files you will need. http://www.blackberryforums.com/ring...alk#post191267
Scoll down to the 8100 section and download the OS you want, and the ALX file that corresponds to that OS.