I doubt if that is possible in AFL since it takes the values set by the system i.e.based on O,H,L,C as per the formulae created by Welles Wilder - I am open to correction here. If you want to use your own values you will have to get hold of a copy of the book by Welles Wilder and write the code in AFL.
R
ps: if you cant get access to a copy of the book let me know and I will copy the formulae from the book and send them to you.