SelectItem default value

[learnsnip]

I have a selectItem which is getting generated through Datasource. It is listing the name of all countries.

'1', USA
'2', Canada
'3', UK

I want to set an additional value from front end for the select item whose id will be '%' and display will be "All Countries". On selecting the All Countries option I would be able to send this '%' as character to my db query.

I want to display the drop down with value map as :

'%', All Countries
'1', USA
'2', Canada
'3', UK

How to achieve this using smartGWT ?

I m using Version: SmartclientPro, SmartGWTPro2.3

[learnsnip]

Scenario:I have a datasource selectitem. I want to add additional valuemap explicitly to the selectItem with an option 'All Countries' with key '%'.
Question: How to apply default key value like the above question to the datasource selectItem or comboBoxItem ?
Even a Hint is welcome.
Version: SmartGWTPro2.3, MYSQL
Thanks,