What do you mean “converting a TGraph to a TH1”? I don’t think there is one clear meaning attached to this conversion: a point in a TGraph has x and y coordinates, a histogram is a collection of bins and their number of entries.
Also, in principle it would be much easier to just fill an histogram as you fill the TGraph 
If your starting point must be a TGraph, then you can loop over all its points and fill a TH1 with those.
Assuming you want to fill TH1 with the x coordinate as histogram value and the y coordinate as weight (not tested):
TH1 h; // the histogram (you should set the number of bins, the title etc)
auto nPoints = graph.GetN(); // number of points in your TGraph
for(int i=0; i < nPoints; ++i) {
double x,y;
graph.GetPoint(i, x, y);
h->Fill(x,y); // ?
}