"*I am trying to create a connected account for credentials which I already have*" — I'm assuming your users have already finished auth flow and you are handling the access tokens and want to shift to Composio but don't want to ask your users to authenticate again? — In this case, you can use the access tokens and inject them using custom auth params:
https://docs.composio.dev/docs/custom-auth-params

But if you want to use a connected account(access-token refresh handled by Composio), you have to get your users to go through the auth flow.
Let me know if this helps.