Yes, you can add it on, although it may make sense to upgrade.
But - managing WS and LP comes with challenges. Depending on how partitioned you want your database, you can manage this with a series of smart list filters.
Add some fields like
Then you'll need suppression lists that everyone respects to avoid sending to the wrong groups.
Other questions to ask
"Would leads be members of more than one brand or BU?"
Is your CRM already segmented this way?
Are all BUs on the same CRM instance?
How are communications managed now?
The answers will help you decide if WS/LP is right for you.