Thanks for asking question! If I have understood right you want to purchase a single wildcard SSL certificate for *.mydomain.com and apply it to multiple subdomains, If so you may want to know that Certificates are specific to Resource Group and are visible to all the web apps in that resource group.
As mentioned in the document "A certificate uploaded into an app is stored in a deployment unit that is bound to the app service plan's resource group and region combination (internally called a webspace). This makes the certificate accessible to other apps in the same resource group and region combination. "
In case your other App service/ app service plan is in a different resource group, then you need to again Import/Upload the certificate.
Let us know.