Hi Lavanya,
do you want to add a user field on the task list header or to the task list operation?
If want to add a field on the operation user data, then you can do this by customizing:
Maintenance Plans, Work Centers, Task Lists and PRTs -> Task Lists -> Operation Data -> Define User Fields
On header level I don't know any user fields - if you'd like to attach your text information to the header, then you could use the classification.
Regards,
Patrick