Not quite sure what you are trying to achieve.
This should work and you could make it an UDF.
DECLARE @Routes TABLE( Location varchar(25))
INSERT INTO @Routes
SELECT 'NY Store1' UNION ALL
SELECT 'NY Store2' UNION ALL
SELECT 'LA Central Store'
DECLARE @MultipleLocations varchar(500)
SELECT @MultipleLocations = CASE @MultipleLocations
WHEN ''
THEN COALESCE( @MultipleLocations, '')
ELSE COALESCE( @MultipleLocations, '')
END + Location + CHAR(59)
FROM @Routes
SELECT SUBSTRING( @MultipleLocations, 1, LEN( @MultipleLocations) - 1) AS '@MultipleLocations'
I wasn't born stupid - I had to study.