At the moment eMember API only allows the creation of a new member account or modify an existing account.
If you need an API to look up data from a 3rd party application then that can be arranged too. Please use our custom job form sumit a query with all the details:
https://support.tipsandtricks-hq.com/need-custom-work-done
Please include what will be your look up key (member ID, username, email address?)