I hope you are already fully aware of the extreme dangers of granting any shell access including jailshell access? I am making an extra effort to point this out as 'rar' is one of the "preferred" compression formats used commonly by hackers, pirates, and script kiddies and though this may or may not be the case whenever anyone uses the words "SSH" and "rar" in the same paragraph, I tend to get a little nervous on that one.
The above said, installing "rar" support is just a simple matter of downloading
the source with wget and compiling it and is pretty much a "no-brainer".
Here is a fairly good article on the subject so I won't re-invent the wheel:
Open rar file or Extract rar files under Linux or UNIX