No viruses exist in the wild for BBs. That doesn't mean that you aren't susceptible to being hacked via bluetooth though. A BlackBerry is a very secure platform.
you can however install jad/jar files manually on your BB
Actually you just need the .jar file, put it on your media card, open the "Media"-app on your BB, open the menu, select explore, switch folders until you reach the folder you put your .jar in and open it, it will show a download dialog but actually just install the .jar you put there.