currently running a 1×10 setup and need some additional teeth for going up hill will I be able to use current chain if I bet an extender or will I have to get a new one

depends on whether you have a long enough chain at the moment and whether said chain is worn.

YOu dont have to automatically change chain when you get an expander but you might need to.

chain and cassette is only.a few 100 miles old

Probably, rather than buy an expander I would just get a subrace 11-42 cassete and a chain. Will have fresh drivetrain for £70 ish

If your chain is sized right at the moment then yes you will need a new chain.

I’d go with Wiggles suggestion and get a Sunrace cassette and new chain.

If yours is still in good nick you might get a few quid for it in the classifieds.

Or add a couple of links back in if needed.