For example, if the number is 123456, I want it to become RNE123456.
Is this what you need?
If you want to change the prefix obviously you can remove RNE and input any prefix string.
Press OK and return to the spreadsheet. It should automatically prefix the rows.
Image of Format Cells dialog
Enlarging column should fix 'E' errors:
Use Custom format
This works also on numbers. You can also type this format under conditional formatting to apply to more creative answers.