Function Problem (Sorry Thats Not Very Descriptive)

Hiya Peeps,
Got a basic update table:

CREATE PROCEDURE CraigSource2Dest AS
INSERT INTO TCraigDest(
MyCertN
,MyResult
,MySpec)
SELECT
TOP 10 dbo.TCertResults.ICertResultsN
,dbo.TCertResults.ICertValue
,dbo.fx_source2dest(dbo.TCertResults.ICertValue)
FROM dbo.TCertResults
GO

As you can see theres a function in the middle of select statement, that returns a value based on what passed into it. The procedure above works fine.

The problem I have is this, can I use the value that the function returns later in the SELECT statement. As an example lets say the table had 4 columns and in the fourth column I wanted to insert the value that the function returned + the word 'TEST'.

I've tried 2 methods of doing this the first :
CREATE PROCEDURE CraigSource2Dest AS
INSERT INTO TCraigDest
(
MyCertN,
MyResult,
MySpec,
MyDerivedColumn
)
SELECT TOP 10 dbo.TCertResults.ICertResultsN
,dbo.TCertResults.ICertValue
,dbo.fx_source2dest(dbo.TCertResults.ICertValue) AS MyNewValue
,MyNewValue + 'TEST'
FROM dbo.TCertResults
GO

When I try this I get the error that MyNewValue is not a valid column name.

And the 2nd method was to try to assign the value returned from the function to a variable. Something like this

CREATE PROCEDURE CraigSource2Dest AS
DECLARE @MyNewValue AS VARCHAR(65)
INSERT INTO TCraigDest
(
MyCertN
,MyResult
,MySpec
,MyDerivedColumn)
SELECT TOP 10 dbo.TCertResults.ICertResultsN
,dbo.TCertResults.ICertValue
,@MyNewValue = dbo.fx_source2dest(dbo.TCertResults.ICertValue)
,@MyNewValue + 'TEST'
FROM dbo.TCertResults
GO

This way gives me ERROR 141 : A SELECT statement that assigns a value to a variable must not be combined with data-retrieval operations.

Bit stumped really up to the point that I don't even know what to search for to look for help.

Any help would be grand.