Since Properties extends Hashtable, can I use the Hashtable methods to add elements to a Properties list?

Technically speaking you can. However, you have to make sure you only add key-value pairs where both are strings. If you add something other than a String, the listing, loading, and saving methods won't work as expected.

Like the Stack/Vector subclass relationship, Properties/Hashtable should be a has-a relationship, not an is-a/subclass relationship.