Is there any way to access and use (read/write) a local(off-chain) file via a Sawtooth transaction processor?
Hyperledger Sawtooth allows one to write transaction processors in multiple languages. It is an independent service running adjacent to a validator node. You could make use of the programming language benefits to do anything you want.
Having said that, it is not recommended to do file IO operations from within a smart contract. If you're planning to use local storage outside the state database, please ensure that you retain determinism. In blockchain world, this is generally done through oracles or another trust network similar to blockchain. Otherwise your blockchain network will start behaving differently every time the external file dependency has changed its value. This is equivalent to being malicious in the network.
Note that the Sawtooth Sabre transactions are wasm based and they are limited in terms of feature set offered through a programming language.