Can we use the LINE User IDs we obtained from a different way?

Yes, you can do it!

After completing the Messaging API channel setting, please store the value of the LINE User ID in the "LINE User ID _ X" property of the contact record. Then, you will be able to send LINE messages in a workflow tool and having a 1:1 chat in the contact record.

We recommend storing the value of "LINE User Name _ X" property and "LINE User Picture _ X" property if possible. Those are used in several parts of the integration. Otherwise, you might see "Undefined" text in some parts.