ID, ServerID, OnlineSince

ImportServerName
NewYork

London
Paris

Tokyo

ID, ServerName
0   Not specified
1   NewYork
2   London
3   Tokyo
4   Munich
5   Salzburg

insert into OnlineServers (ServerID, OnlineSince)
select
(
    select ID
    from ServerLookup
    where ServerLookup.ServerName = ImportServers.ServerName
       or ServerLookup.ServerName = ''
),
GETDATE()
from ImportServers