Conditionally insert a new row into a data flow

I have a data flow which I have sorted into a required order using the sort tool.

When there is a change in the value of a particular column from one row to the next, I would like to insert a new data row between the two rows and set the value of each column in the new row, for example to a fixed value or a formula using values from the previous or next rows.

Any ideas?

Thanks. Looking forward to Inspire Europe in London, September.