I think you would go in way over your head trying to create an Excel spreadsheet object from RPG.
You have a couple of options here, and since you are really creating an ASCII/Tab-Delimited file and not a spreadsheet the answers are pretty much all in Excel.
1. Create a spreadsheet that downloads it's data directly from the AS/400 viaan ODBC data driver, Client Access has one. Set up the resulting data fields to conditionally format a cell based upon it's contents.
2. Create a macro that reads the tabbed file and then conditions the formatting of the cell for each record you read in.
I have done both and I think #1 is easier.
I have created an ile rpg program that creates a Microsoft Excel spreadsheet(tab separated) from an AS/400 file. The users love it, but(and you just knew there would be one) they would like me to highlight(BOLD) certain cells if they exceed certain conditions. I have tried highlighting(BOLD) some fields in an existing spread sheet, copying that from the IFS to a database file on the 400 to see if I can find the control codes that cause highlighting(BOLD) in Excel. I am not sure I can do this. Does anyone have any experience with such things?