Hi Max,
Sorry to hear about your Dad, I hope he has a speedy recovery.
In terms of your question, it is unlikely that your Mum and Dad's Mortgage lender will allow them to borrow more at their current age, but its always worth asking. Each lender has a different lending criteria. You can ring and just ask what the maximum age they can take any customer up to is. You don't need to be on the Mortgage to ask that
In terms of you being added to the Mortgage, it is unlikely to help because most Mortgage lenders will go off the oldest persons age on the mortgage rather than the youngest so the very fact that your mum and dad are on the Mortgage will mean your age is unlikely to count for anything.
There are some lender who offer life time mortgages, this may be an option, but you do need to be very careful with this type of contract and if unsure seek advice from an IFA
Of course, the other thing Max is to get in touch with your Mum and Dad's local council. It may be because of your Mum and Dad's age and your Dad's illness and reasons for needing to make these changes to their home that they qualify for financial assitance with the modifications. Always worth checking and of course would mean they didn't have to borrow anything to do it.
I really hope this helps
Best wishes
Sean Hughes
http://www.ifacompare.co.uk/