Converting String To Unicode String In T-SQL

Hi,

We have stored proc name proc_test(str nvarchar(30)). So far this proc
has been invoked from a .NET application assuming that only English
character strings will be passed to it. The calls are like
proc_test('XYZ')
We now have a requirement for passing Chinese strings as well. Rather
than changing the calls throughout the application, we would like to
handle it in the stored procedure so that it treats the string as a
unicode string. Can we apply some function to the parameter to convert
it to unicode so that we don't have to call with an N prefixed to the
string?

Thanks,
Yash