It’s not directly possible from WHM inteface. But you can manaully edit the cpanel config files and assign dedicated IP for the addon domain and then install the SSL
Edit the file
Code:
/var/cpanel/userdata/username/addon-domain.com also checkl file /var/cpanel/userdata/username/addon-domain.com_ssl
and change the IP to a dedicated IP. Also edit the DNS entires for the addon domain and set the New IP there.
Now rebuild the apache configuration files
Code:
/scripts/rebuildhttpdconf
Now you can directly install the SSL from WHM.