Finding all users with a matching attribute using Okta Expression Language

I have two third-party applications installed in okta. I would like to sync a custom_attribute _1 (guaranteed to be unique for every user) from application 1 to another custom_attribute_2 in application 2. Is it possible?

Essentially, I want to create an Okta Expression that can do the following:

  1. Among all the users synced into Okta from application 1, find the specific user with a matching custom_attribute_1.

  2. Map the email attribute of the specific user to custom_attribute_2 in application 2 in Okta.