If you want to do it only through repeater only option i can think of is building the repeater code dynamically in the code behind and redering it on browser. There might be other ways i am not sure !!!!!
If you would like to use datalist adding columns will be easy through Datalist.repeatcolumn property.
Thanks,
Surya Prakash Jayanti
http://suryaprakashj.blogspot.com
http://jnsuryaprakash.blogspot.com
http://sqlserver-qa.net/blogs/dotnet_40/default.aspx
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