See for instance here:
where you would use TTree::Draw (e.g. tree->Draw("2*x+3*y"), save to a TGraph and then fill the new branch from the graph points (or perhaps directly fill the new branch from the GetV array/s, without going through TGraph). The arithmetics part is fast, but doesn’t directly save to a new or existing branch.
The link Rene Brun gave doesn’t seem to work anymore but it’s here (scroll down to the section he mentions):
https://root.cern/doc/master/classTTree.html#a73450649dc6e54b5b94516c468523e45