question about nopcontext.Current.Session

I want to add a property to customer session.
I don't want it to be changing unless I change it.

I want to add LastLockedPrice to Customer Session

I added it to CustomerSession.cs file
I tried this
NopContext.Current.Session.LastLockedPrice = DateTime.UtcNow;
but it doesn't save it.

I will appreciate if anyone can reply how to add a property to customer's session and it doesn't overwrite.