Unfortunately non-coded steps cannot create/define new variables at run time, not even in a loop. They can only be defined at design time. Only coded steps can inject a new variable with a new name.
What I would do in your place is to do it all in code (keep in mind I am very comfortable working in code, but I realize not everyone is), i.e. use non coded steps to navigate to my website, login, step forward to the page that displays the data, then in a coded step extract all the data and write it to disk all at once.
The problem with trying to use a standard loop step is that they must run for a fixed number of times. Since you don't know the number of rows that will be present from the start, the loop will not meet this requirement. In a coded step we can detect the number of rows that are present and much more easily extract all of the data from all of the rows.
One catch I must warn you about though, does the order of the data matter when you save it? We have found that Silverlight doesn't always store the data in the same order that it is being displayed in. Sometimes it actually maps between a row of data and where it is placed on the screen. This can be a killer if you're trying to validate the sorting order of data.
To help get you started I have attached a sample test I put together a while ago that performs actually scans and verifies the sorting order of multiple columns of a RadGrid. I will admit that at quick glance the code looks a bit scary. It was necessary to overcome the peculiar way Silverlight stores and displays rows of data. You should be able to reuse most of the code to achieve your goal.
the Telerik team