It is possible to add a comment by performing a transition on an issue. Is it also possible to add a comment without to perform a transition via the REST API?
JIRA 5's REST API will have support for this through the Issue resource (http://docs.atlassian.com/jira/REST/5.0-beta3/#id1781065)
There is no method for adding comments alone as far as I can see!
JIRA Command Line Interface has actions for both situations, but those uses the SOAP API.
Confirmed. There is no way to add comment to an issue via REST API in JIRA up to version 4.4 (other than while performing the transition as you mention).
I hope though that JIRA 5.0 will finally offer such fundamental feature.