Using the result of an expression (e.g. Function call) in a stored procedure parameter list?

You need to use an intermediate variable. SQL Server does not support this kind of operation in the parameter list itself though it has been on the TODO list for a few years! (See Connect Item: Use scalar functions as stored procedure parameters)

The grammar for EXEC is

[ { EXEC | EXECUTE } ]
    { 
      [ @return_status = ]
      { module_name [ ;number ] | @module_name_var } 
        [ [ @parameter = ] { value 
                           | @variable [ OUTPUT ] 
                           | [ DEFAULT ] 
                           }
        ]
      [ ,...n ]
      [ WITH <execute_option> [ ,...n ] ]
    }
[;]

The documentation is not currently that clear on an acceptable format for value but it seems to be only "simple" expressions such as literal values or @@ prefixed system functions (such as @@IDENTITY). Other system functions such as SCOPE_IDENTITY() are not permitted (even those which do not require parentheses such as CURRENT_TIMESTAMP are not allowed).

So for the time being you need to use syntax such as the below

DECLARE @pID INT;

SET @pID = 1;

/*If 2008+ for previous versions this needs to be two separate statements*/
DECLARE @string VARCHAR(50) = 'Could not find given id: ' + CAST(@pID AS VARCHAR(11))

EXEC WriteLog
  'Component',
  'Source',
  @string 

DECLARE @pID int;
declare @IdAsString varchar(100)

SET @pID = 1;

Select @IdAsString ='Could not find given id: ' + Cast(@pId as varchar(10))

EXEC WriteLog 'Component', 'Source', @IdAsString

As pointed out by Martin, the following only applies to columns not variables.

Note that I have amended your cast to varchar(10) this will allow for integers larger than 1 digit. varchar will only allow 1 character