The question really asks it all. We have a bunch of really old S/36 programs that need to be moderized. I am more looking for articles or sites that explain what the old method does and how to convert it to RPGLE.
Here's a manual for RPG II on the AS400 so you can understand the old code.
You could just trying to compile the code as RPGIII using CRTRPGPGM. Fix the compile errors on the listing to convert the code to RPG III.
Once you have RPG III source you can continue further to convert it to RPG ILE with the CVTRPGSRC command.
It's might also be possible to take the RPG36 code directly into CVTRPGSRC to convert the source to RPG ILE.