declare @clients table  (nomer varchar(10), seria varchar(10))
insert into @clients(nomer, seria) values
('N1', 'S1'),
('N2', 'S2')

select 
	(select '@DOC_DATE' = convert(varchar(10), getdate(), 120), '@DOC_TIME' = convert(varchar(8), cast(getdate() as time)) for xml path ('DOC_REQUISITIES'), type)
	,(select '@DocDate' = convert(varchar(10), getdate(), 120)
		,'@FirmID'='0676899'
		,(select '@attribut' = row_number() over (order by c.seria, c.nomer)
				,(select '@attribut'=1
					,(select '@attribut3'=54
						,(select '@attribut4' = 128 for xml path('CLIENT_INFO3'), type)
					for xml path ('CLIENT_INFO2'), type)
				for xml path ('CLIENT_INFO1'), type)
				,(select '@attribut3' = concat(c.seria, ' ', c.nomer) for xml path('CLIENT_Passport'), root('CLIENT_INFO4'), type)
			from @clients c 
			for xml path ('CLIENT'), type
		) for xml path ('CLIENTS'), type)
for xml path ('ROOT')