Can we use DigiByte in a system similar to ethereum so that DigiByte is used as a fuel (gas, payment) to do smart contract? Maybe it is not necessary as that market is occupied by Ethereum already.
This would be basically impossible to implement as far as I understand it.
Not impossible, just not currently implemented in this way so not possible as things stand, I would have thought here.
In the fullness of time, if smart contracts become necessary to compete in the cryptosphere, I'd expect DigiByte to be able to compete in this market. The possibilities are endless but I'm sure DigiByte developers are working on implementations that aren't currently being explored by other block chains, for the time being anyway. Doesn't mean someone else won't knock up a version using DigiByte on their own just because they can though, after all this is all open source.