Not quite. You have to put the word into the genitive case. In the case of curia, court, you do this by changing the -a to -ae. In the case of Epicurus, you would do this by changing the -us to -i. Unless it declines like cantus, in which case you would change the -us to -ūs, or unless it’s considered a Greek word and thus immutable.
(The rules for how you decline loan words are complicated and change over time, and I don’t remember off the top of my head how they do that, so I don’t know what to recommend.)
If you just want something acceptably fake in dog-Latin, ‘amicus Epicuri’ is probably close enough.