You will have to use an INSERT..SELECT option

INSERTINTO [PLSy]( <<Column1>>, <<Column2>>, ....<<Columnn>> )

SELECT<<Column1>>, <<Column2>>, ....<<Columnn>>

FROM[srv5].[DB].[dbo].[PLSo] -- Navision

WHERE[timestamp] NOT IN ( SELECT [timestamp] FROM [PLSy])

Another better way would be to use NOT EXISTS instead of NOT IN

INSERTINTO [PLSy]( <<Column1>>, <<Column2>>, ....<<Columnn>> )

SELECT<<Column1>>, <<Column2>>, ....<<Columnn>>

FROM[srv5].[DB].[dbo].[PLSo] AS Nav-- Navision

WHERENOT EXISTS( SELECT * FROM [PLSy] AS PL WHERE PL.[timestamp] = Nav.[timestamp] )