well based on how i read your description, it sounds like a simple calcuated column could be used;

ALTER TABLE [dbo].[GV_STNDetails]ADD STN AS RIGHT('0000' + CONVERT(VARCHAR,[From_StoreCode]),4) +RIGHT('000000' + CONVERT(VARCHAR,[STNID]),6) PERSISTED

if you run this example in tempdb, is this giving you the results you are looking for?

/*

--Results

STNID CourierName AWBNo DispatchedDate STNNO From_StoreCode TO_StoreCode STN

----------- ------------------------------ ----------- ----------------------- -------------------- -------------- ------------ ----------

1 FedEx 14 2013-04-17 07:22:26.040 PO-14 5 6 0005000001

2 UPS 42 2013-04-17 07:22:26.040 PO-15 1 1 0001000002

*/

CREATE TABLE [dbo].[GV_STNDetails] (

[STNID] INT IDENTITY(1,1) NOT NULL,

[CourierName] VARCHAR(30) NULL,

[AWBNo] INT NULL,

[DispatchedDate] DATETIME NULL,

[STNNO] VARCHAR(20) NULL,

[From_StoreCode] INT NULL,

[TO_StoreCode] INT NULL,

STN AS RIGHT('0000' + CONVERT(VARCHAR,[From_StoreCode]),4) +RIGHT('000000' + CONVERT(VARCHAR,[STNID]),6) PERSISTED)

INSERT INTO GV_STNDetails(CourierName,AWBNo,DispatchedDate,STNNO,From_StoreCode,TO_StoreCode)

SELECT 'FedEx',14,getdate(),'PO-14',5,6 UNION ALL

SELECT 'UPS',42,getdate(),'PO-15',1,1

SELECT * FROM GV_STNDetails