Shimano Tiagra – Expanding Cassette??

Hi,

I have a 10 year old roadbike that is kitted out with a Shimano Tiagra groupset. It is 50/34 on the chainwheel and 11/28 (10spd) on the cassette. It is in pretty good nick but  don’t really want to spend lots of money upgrading ( I would rather put that towards a new one when the time comes). I have a trip coming up with a few tasty gradients over a few days. So question: Is there any way I can expand the rear cassette to incorporate a bigger granny cog, say a 32t (or a 30t), without having to buy a new groupset / lots of new parts? Is there something similar to the ‘oneup’ system where I can take one sprocket out of the current cassette and replace with a 32t sprocket in the cassette? Or can I replace with an 11/32 cassette only (if I can get one) withut buying a new shifter/derailluer/chain etc? I understand the issue with the cage length, but if I only used a new 32t sprocket whilst in the 34 chainwheel upfront (and adjusted tbe B screw) would that work? Or am I just barking up the wrong tree?? Just trying not to spend a lot of money.

Many thanks in advance

Cheers, Jason