I meant that it might be easier to start from a plain, rectangular partition layout. Since you seem to want a mix of sunburst and treemap, temporarily removing the radial arrangement might allow you to better understand the layout’s behavior and how you need to adapt it.
Please note the second part of the sentence that you quoted:
[…] and instead accumulate/assign each node’s
.valueas you please.
node.sum() recurses through all children of a hierarchy and adds the sum of their values to the parent node’s value.
d3.partition merely requires that a node’s value isn’t smaller than the sum value of its direct children. Wether you let d3 handle that by calling .sum(), or sum up the values yourself, that’s up to you. 
You can base your adaption on the original implementation.