I just realized there's a section in the manual for this!
When I try
apt-get source pure-ftpd
it says E: unable to find a source package for pure-ftpd
I think I may need to add something to the sources.list, but I'm not sure which one to add :/
EDIT: I was correct. To others who might have that problem, make sure you have these sources added to your sources.list:
lenny main contrib non-free
Problem solved! - The steps on how to build it is in the ISPConfig 3 Manual.