Custom field inside stream

I apologize for all the questions regarding custom fields (it's just a very large aspect of the site I'm working on).

What I'd like to know, if possible, is how to add a custom field before the stream pulls the username.

So for example, the stream currently shows this:

random_user just logged in to the site. — 34 minutes ago

What I'd like it to show is:

CUSTOM_FIELD (random_user) has just logged in to the site. — 34 minutes ago

Thanks again for all the help and support so far, and the great product.