I have an existing .wallet on my PC. I want to use this with MultiBit HD. Is it possible to use this wallet instead of creating a new one?
This is because the two versions use two different types of key management systems.
Multibit HD is hierarchical deterministic key management. All keys generated can be recovered from a seed since the keys are generated deterministicly.
Older multibit clients used single key management where you could not recover all keys unless you had a back up of your wallet or private key.