You cannot, I tried, then checked with HipChat support, who confirmed it isn't an available functionality. So I opened this suggestion ticket:
Please vote for it so it will get more attention from the development team. You may also comment to emphasize your use case.
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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