var listA = ...;
var listB = ...;
listA.ForEach(m =>
{
var s = listB.FirstOrDefault(n => n.StoreId == m.StoreId);
if(s != null)
{
m.StaffCount = s.StaffCount;
}
}
);

listA = (from a in listA
join b in listB on a.StoreId equas b.StoreId into ab
from ba in ab.DefaultIfEmpty(new ModelXXXB())
select new ModelXXXA
{
StoreId = a.StoreId,
OrderCount = a.OrderCount,
StaffCount = ba.StaffCount
}
).ToList()