This is an interesting question, but I'm not exactly sure what it is.

If the aim is to insert semicolons (or other character) in the raw data from the "suppose I have a program", prior to Excel getting the data, it's probably a question that should be asked of that particular program.

If the aim is to accept the raw data as is and have Excel manipulate it before placement of the data into cells, we would need to have an accurate sample of the data Excel is getting to work with. I emphasize accurate because if the data is now going into separate cells then the 12345 as you indicate would have a delimiting character between the individual digits.