Hi @ALY ,
Welcome to our forum!
is there a way we can import the client's certificates to our GAL
As i know, we can only publish the certificates to GAL but not configure the certificates for all users' client. More information: How to Configure S/MIME in Office 365.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.