How can we custom define custom tables to use with our extentions?

Your question is not clear.

When you create an extension, you have to create a manifest file (xml) where you describe all the files that must be installed and also the MySQL tables that must be created.

So if you are have a package of your extension, you can install it joomla.
As this is a specific extension it will not be recognized by the JMS tool.

In this case, you have to go in the slave site and re-isntall the extension to let it creates the MySQL table that are defined in its manifest (xml).

So just install the extension in the slave site.

Ex I want to specify certain tables within Jomsocial, I want to share users but not events

If you want to create specific sharing rule, this is not directly possible.

The only solution is that I create a specific plugin for you that would let you declare the specific rule. Such kind of plugin can be create in 1 hour.
www.jms2win.com/en/joomla-multisite?page...35&category_id=1

Becarefull when defining a rule that you must ensure to keep a data model consistency.
This is the reason why we do not provide sharing for all the extensions.
So you must be an expert when doing such specific rule.