It depends on which UC- they each have their own appeal process. For instance, UCSD only allows an on-line appeal that is limited to 250 words and they will not accept any supporting documentation or supplements. You need to look at the appeal process for the UC you are interested in and follow their directions exactly. They will all throw out your appeal if you do not follow directions. Whatever you submit, it will have to be new information that was not included in your original application. If you have not previously discussed your medical condition and it impacted your performance in HS then you may want to use that as the basis for your appeal. Good luck!