you can get close to it if your use does not require high drive currents, by treating hte transistor as an open collector drive and having a weak pullup on it you can get the situation where, when the transistor is off its collector will go high due to weak pullup but any other device on that line could control it (Pull it low or allow it to also stay high or even add to the high with its own weak pullup.)
This is how many systems share a connection to a device by using a common open collector drive. It is not quite the same as tri-state but comes pretty close.
If you ask yourself why you need tristate then the answer is usually to allow something else to control the connection, or at a minimum pretend your not connected to it. This is how I2C works, you can have upto 127 devices all on the same pair of wires, as there all open collector drive and there is a common pull up, the system still works and devices no participating in driving the piins are easily able to listen to the same pin, effectivly making it an input without more complex circuitry
