Can I extend my stay in Thailand using a border hop after my initial 60-day visa?

You can only extend once

This new ruling favours people that are near land borders. :)

Can you extend at immigration office for 60 days or is it only 30 ?

I think what you're asking about is a 60 day visa exempt entry <- where you show up to thailand without having bought a visa before you came and get stamped in FREE for 60 days.. It used to be 30 days but on July 15 it went to 60 days

You can definitely bounce out and back at the Nong Khai border with Lao to get a new 60 day visa exempt entry and extend that 30 days if you want to at the immigration office for 1900baht