I know that this is not the answer you are looking for, but the only way to do this is to declare a local variable, assign it a function value and use it as a parameter value:
DECLARE @input nvarchar(255) SET @input = a_function_that_returns_string('abracadabra') EXEC stp_dummy @ input=@input
With SQL Server 2008, this can be done in 2 lines:
DECLARE @input nvarchar(255) = a_function_that_returns_string('abracadabra') EXEC stp_dummy @ input=@input
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