Mmm... I just noticed they use two IPs per host, one that they use to resolve all the names hosted in their server, in your case, istore.za.net resolves to 216.245.223.218, and another IP that seems to be used only to resolve the actual host name, in your case lotus.istore.za.net resolves to 216.245.223.219.
So, to use their certificate (which will give a warning like I said), you would need to use a third-party DNS server, and make it point to 216.245.223.219 instead of 216.245.223.218.
Of course, setting up a DNS involves a little more than this, like setting MX records and changing the domain's nameservers at the registrar. Very simple actually, but can be a bit overwhelming at the first time, but if you wanna try, there is sitelutions.com which is free.
Now I'm not sure why they use this setup, but perhaps they don't want their certificate being used with other domains names (even through browsers give a warning).
As for the IP, getting one completly by yourself is complicate, and impossible, because they are sold in bunchs (networks). I'm not sure if there are service that sublocates single IP's, but this road is more complicated.
If you want to use your certificate, you to move to a paid hosting service that already includes a dedicated IP in the package, or that offers it as optional.
But with some luck, since you already has a certifacate, if x10hosting has a spare IP, and you get some admin in a good mood, perhaps you could get lucky enough to have them to set it for you. Did you ask them about using your certificate? Did they give your a answer?