Thank you for posting this in Microsoft Q&A.
I understand you want to populate TAP expiration date in lifecycle workflow email customization.
Yes, Unfortunately, there is no way to populate the expiration date of a TAP in the in-lifecycle workflow. The only way we were able to fulfill your requirement was by using a custom extension.For your reference: https://janbakker.tech/automate-issuing-temporary-access-pass-for-joiners-with-lifecycle-workflows/
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya.