Multiple Choices With Parameters

I need to pass a parameter to a stored procedure that allows the user to select all store numbers or a combination thereof.
The portion that selects all works fine. I am have trouble with the later. My code looks like this:

CREATE PROCEDURE vch_GetSurgeryPatientsWithoutOrdersFromEyeSite_clb
(
@StartDate datetime,
@EndDate datetime,
@center varchar (1000)
)
AS

IF @StartDate IS NULL
BEGIN
Set @StartDate = GetDate()
Set @EndDate = (GetDate ()+90)
END
Declare @StoreList TABLE ( CenterID int )

-- Get our Centers
IF ( @Center = 'ALL' ) or ( @Center IS NULL )
BEGIN
INSERT @StoreList
SELECT DISTINCT POS_Site_ID FROM LVIGP.dbo.POS40108 (nolock)
END
ELSE
BEGIN

INSERT @StoreList
SELECT DISTINCT POS_Site_ID
FROM LVIGP.dbo.POS40108 (nolock) WHERE POS_Site_ID = @Center
END

the first if statement returns all of the store number like this
CenterID
001
002
010
024
057

the second if returns nothing
I tried using coalesce but it returns the store number like this:
CenterID
001, 002, 010, 024, 057
This does not work with the rest of my program.