Yes there is, but only contacts with Messenger Plus! installed will be able to see it. Adding the code for bold to your name works in a similar way as putting colour in it.
To make your name bold just open a conversation window, type /nick and a space and press ctrl-b. Then type your name and press enter. You can also add other formatting codes such as colours in a similar way; instead of ctrl-b you need to press ctrl-k and click a colour then.
Make sure you do not try to copy/paste the name to the Options dialog window, just type this in a conversation window and press Send. Also make sure that you type a space after /nick
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