You're going to find that you actually need users to be in liferay. It is the only way foreign keys to users can be supported.
Trying to cache so many objects in memory on any kind of system can be a hugely wasteful use of resources. Unless all of those users are logging in on a regular basis, you'll going to have a dedicated pile of crap (yes, it will smell like crap if you get close to it) most of which is rarely needed or used.
Lots of orgs want to use their own user table, not copy/load into Liferay, but that quickly breaks down because the external system doesn't have the fields that Liferay wants to populate and Liferay won't work without being able to satisfy the foreign keys. Your Expando tables won't work for the users, you won't be able to determine who owns what, the Liferay permissions (all based on the Liferay users) won't work, group/site/team membership won't work, you get the picture?
Your best bet instead is to ask how to store so many users in the Liferay environment and still make it work. The answer here is to use whatever sharding mechanism your database has to effectively handle so many records, and replace lucene with Solr so you can handle all of that data.
