Thanks luigi, but it does not work for my characters, see here:

\definefont[tradoa][name:trado*arabic at 20pt] % or file:amiri-regular.ttf*... file:alq.ttf

\directlua{ local function replace_txt (buffer)

buffer=string.gsub(buffer,"اللهِ","اللَّـه") return buffer end
callback.register("process_input_buffer",replace_txt)
}
\starttext
my text {\setupalign[r2l] \tradoa اللَّـه}
\stoptext

"trado" ist just the font "traditional arabic", nearly every system have it, so you should be able to compile this. It does not replace the character.