Not at this time out-of-box, however you can install Rainloop to a standard web domain and use it that way (which is the easiest way). Alternatively you should be able to modify the webmail templates accordingly, but we will not provide official support for this at this time.
We do have an updated webmail experience based around Roundcube 1.4 in the works, however I don’t have an ETA to provide right now as to when it will be ready to ship.