I use several tools for building SFZ files
SFZ'ed
http://audio.clockbeat.com/sfZed.html read more here
http://www.sfzformat.com/index.php?title=Main_Page some of the tools I use you cant get anymore as even the OP codes are not on cakewalks DEV Exchange website anymore.
but they are easy enough to edit and using a notebook editor speeds things up as well.
theres a lot of copy/pasting.
ive built several kits from the ground up with my own drum kits.
its pain staking and can take hours but if you have the samples already (velocity samples)
its pretty easy from there, just takes some time.
a side note, you can drag + drop almost any kind of sample (WAV, FLAC, EXC) right in to session drummer and it will trigger it for you.
Example:
Grab a kick sound sample and drag it to the kit drum. hover your mouse right over the drum and release the mouse button.
your sample is now loaded.