Yes it is, but it is very very hard to do do this. Sometimes the way the glass is made, it has to be made in steps. So if a perc breaks inside, it can be very difficult to get the pipe where it needs to be in order to fix the perc.
Each perc and break has its own issues on the difficulties. It is always best to send a picture to firstname.lastname@example.org. Jeff can help you out better with this.