I have a T-Mobile UK 8310, OS v. 4.2.2.176, which is a multi-language version. While I'm able to read emails, texts and webpages in Hebrew, Arabic, and Russian, my input language options are only English, Russian, and Arabic. Any ideas how can I add the ability to select Hebrew as the input language?

Do you have your OS loaded to your PC? If so, run App Loader and see if those choices appear. If they are UNchecked, check them for Input support.

hmm... not quite sure how to have the OS installed on my desktop (?) I have the Desktop Manager installed, but this is none of the options there, so I'm a bit puzzled - can you please advise on this?

The OS that is on your BB should be downloaded from RIM or your carrier BIS site and installed to your PC, as if you were going to upgrade or install that OS to your BB.
Where can I log into my BIS account? - BlackBerryFAQ