how to run a Step Request for each file located in a folder

It depends what is there in your xml file.
If there is a series of nodes and tree structure like below in your xml file you can just read the whole thing at the parent level from xml file and populate the xml structure at that node.

<1>
<2>
<3>
</3>
</2>
</1>

If this is what is there in your xml file, then you can read the complete structure at xpath <1> and populate the same as a child node in between your original xml request.

You don't need to delete the nodes if you want to keep the default values.