It can be done, but if the only reason you are putting them in a separate file is to limit access to them, this is not needed.
To access data in a table from an external file, you would first add a Table Occurrence for that table in manage | Database | Relationships. You can click the bottom far left button to start a new occurrence box and use "Add filemaker datasource" in the drop down of the dialog that appears to find your external file, open it and select a table from it. Once you have a table occurrence for that table from the external file, you can select the "use values from a field" option and select the table in your external file just as though it is a table defined in your original file.
But since you asked:
...how do I lockout / hide tables... so that users on Ipads will not see those tables and be able to change them
You really do not need to use an external file. You can use layout setup or Manage | Layouts to remove a layout from the layout drop down. This restricts all users from accessing that layout unless they are a full access user and pull it down while in layout mode.
You can also use Manage | Security to give your FM GO users a privilege set that denies them access to the layout, the table, the ability to edit data in the tables...
Excellent...thanks for the info...i'll will try the various options you've listed...i'm sure one will work perfect