It is very simple. All he has to do is himself request witnesses. If he does this, 2 things will happen:
1. The dynamics among republican senators (at least enough of them) will change. The “moderate” ones will have hard time ignoring the Chief Justice who is a conservative appointed by a Republican President.
2. A few republican senators will feel compelled to vote for witnesses and will refer to the Chief Justice to their constituencies who support Trump under any cost. I believe in that case it will be more than 4 senators and we will end up hearing at least from Bolton.
Of course, that’s not the traditional role of the Chief Justice in impeachment proceedings. But we live in extraordinary times. This is the time for extraordinary people, like Justice Roberts, to step in and save the country. No matter what the process is, nobody can stop Justice Roberts from asking for that in the senate.