However, as you're OnDemand, you can't install plugins, so I think you're pretty stuck
Just to add to what Nic has said. If you were not on the OnDemand, then you could look at using a 'scripted field', which I have used in the past to take bits of two fields and combine them to make a thried field. I believe that the plugin is the scriptrunner (verify before using, because I may have the wrong plugin name here)
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