What are you looking for exactly? If it is a specific string that the user will always need to look for, you can build a query in the excel reports area and extract the data there.
Creating a new field just to duplicate comments data seems a bit excessive when you have other options available.
While I have been working for Businesses that are Vendor partners with HP, IBM and Microsoft, my opinions and advice is my own.
The solutions provided are either sourced from my own scripting libraries or from a quick Google Search.
In QC 9.x, you can enable text search, which search through the descriptive fields such as defect description and comments. You may be able to use it instead of trying to sort the field. However, text search is available only with some databases.