asterisk users - Dec 2004 - setting up conference room option

hi;

can i through an existing phone call into a conference room.  For 
example of A calls B (say two fxs dump POTS terminals).  Is there an 
option that will allow a third part say C ( a PSTN terminal) to join 
their call if he/she has a conference room number and the password in 
which the original call was thrown into?

thanks
m. smadi