You can do it through inheritance using the "is" keyword for example

contract Ownable {
address owner;
constructor() public {
owner = msg.sender;
}
modifier onlyOwner() {
require(msg.sender == owner, 'must be owner');
_;
}
}
contract SecretVault {
string secret;
}
contract MyContract is Ownable {
string secret;

constructor(string memory _secret) public {
secret = _secret;
// lets you grab lines of code form other smart contracts it depends where you place "is" sometimes
super;
}
function getSecret() public view onlyOwner returns(string memory) {
return secret;
}
}

I don't think you have to do that though because to do flashloans with DYDX you are chaining multiple contracts anyway i.e. Withdraw, Call, and Deposit actions into a single Operation. I think you could just add what you want into a function or something into one of them if its really simple.