Sub rooms aren't something supported by the HipChat UX, but there have been a few users who have requested similar things before. If it's something you'd find useful, please check out https://jira.atlassian.com/browse/HCPUB-2556 to vote / watch so you can be informed of any updates to it. Feel free to comment on it explaining your use case as well... details like that are super helpful in understanding the underlying need that makes the feature desirable.
...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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