Keycloak Authorization with other system's account

Question

I'm creating public API for other system can get data from my system. so, to authenticate I uses Keycloak server.

Ex: My system has an account tonystack, pw: 123456 (Actually, 100000 accounts) But this account is not exist in Keycloak's user

enter image description here

How to authenticate this account by Keycloak ? Or How to map this account with Keycloak's user account ?

(Sorry, My english is not good for explaining)