Is there a way to use the Block maneuver while wielding a two-handed weapon?

No. You have to be holding a shield in one of your hands to use Block, and if you're using a two-handed weapon that means you have no free hand to use the shield with. Having a shield "equipped" means that you have it in your hand, not on your back.

Or allow a buckler to be used at the same time, but add 1 challenge dice to all attacks with the twohanded weapon...

it is called Parry

Block action requires a shield to be used