Can we use a hook_update_N for a module without hook_install?
No module needs to implement every hook. You can use hook_update_n() and not have a hook_install() function just fine.

You'll probably at 1 point want to define hook_schema() for these tables. An easy tool to get the "drupal" schema for any mysql table (already defined) in your mysql database is the Schema module.


  
  Schema documentation: hyperlinked display of the schema's embedded
  documentation explaining what each table and field is for.
  Schema
  structure generation: the module examines the live database and
  creates Schema API data structures for all tables that match the live
  database.
  Schema comparison: the module compares the live database
  structure with the schema structure declared by all enabled modules,
  reporting on any missing or incorrect tables.
  
  
  Note for MySQL users:
  The Schema module requires MySQL 5. Prior versions of MySQL do not
  support the INFORMATION_SCHEMA database that the Schema module uses to
  inspect the database.