How about trying:-
insert into [stats](StatDate,SiteID,SiteName,StatType,UniqueHits,TotalHits,JoinHits)
select *, JoinHits = (select count(*) from referer where refdirection='OUT' and RefType=#TempItems.StatType and refdate=#TempItems.StatDate
and (siteid=#TempItems.SiteID or siteid is NULL)...