Hey so I only created this account to comment
I wanted to find a nice little program to do this all for me but I did it the long way.
This is the method I used:-
I have a program called 'Sonic Visualiser'. I recorded a piece of L's alias voice from one off the episodes in Audacity and then imported it into SV. There is a setting within this program to view melodic spectrum analyzer. With this view, you can see at what frequencies the voice is split into. There are 3!. A low under tone, a mid and a mid high. You can do all this so far as these programs are free but next I used a software called 'Celemony Melodyne Editor'. This program is souly audio sample based. It allows you to alter pitch, timing etc... I then recorded my own voice saying the same as the recording of L on Audacity and imported it to Melodyne 3 times. I then pitch shifted my first down a whole octave. This is the low part. The next I used a 'throat size setting'. I increased this setting so it was by about 6 notes above the original and then did the same for the 3rd one only increased the throat size up by 9-10. Playing these parts all together gives the sound of L!. (A throat size setting is how they made GLaDOS's voice.). I bought my copy whilst it was still being sold but since it no longer is, I'm sure their is a copy on TPB.
I have no programming knowledge so I can't make a program that does this in one process. I'm sorry. But this is the method I used. If you still want to make L's voice then you can find cheaper, if not free software that does exactly what I did. Hope this helped. Sorry I can't do more. My version isn't dead on because I didn't add enough low to the mix but you get the idea 
(L's voice I used
http://www.megaupload.com/?d=WZ52ZFUL
(My version
http://www.megaupload.com/?d=PM95BI3E