The header file provided by Altru for membership cards may not contain all of the merge fields that you need, or you want to add/modify fields in the header file.
This exact functionality is not something that Altru can currently do.
We have an idea on our Community regarding this: https://altru.ideas.aha.io/ideas/ALTRU-I-1576. We encourage you to vote on this post as this is an opportunity to have a voice in the product development process. Our Product Management team uses a variety of methods to determine new features in Altru, and the ideas in the Community is one way they do this, so your vote helps Product Management prioritize the new features that we put into the product.