Najm,
I think I understand what you are getting at. You want to add a column that looks up from another columns list items? If I have understood correctly you need to:
1. Open thelist in question
2. Modify the settings and columns
3. Add a new column, and choose lookup as the column type.
4. REference the same list and choose the column name.
Yes it is also possible to change a single text item to a choice column. If you just click the column to edit it whilst in settings. Not sure if it can be a lookup though. If not, delete it and add a new one. If you already have data, then export it to a spreadsheet first!
Hope this helps.
Brett
www.combined-knowledge.com
Brett Lonsdale
Partner/Trainer
Combined Knowledge
MCT, MCSD.NET