Usually retemplating controls are the key to solve almost all the problems on Silverlight controls, including Infragistics components.
But I have two questions
1) Since I need to know the structure to retemplate a xamlGrid, there is a "map" or a guide explaining all the layout of a xamlGrid? So then would be easier to work on that
2) Because I'm not sure about what I have to change, I would like to know if it's possible to have a column before the "plus/minus" icon, which is not affected by nested grids. Example:
(my xamlGrid is v2010.2)
Our help includes a Designers Guide which shows the control composition of each control:
https://es.infragistics.com/help/wpf/designers-guide
Specifically for your question, you will want to work with the ExpansionIndicatorCell control.
Devin