Hi Thys,
xFiles is tightly linked to structures, so no it's not possible to have a "structureless" form of "save".
I guess one approach though would be to use the Dynamic File Driver. You could create a dynamic file (Memory driver maybe?) at runtime and then pass that to xFiles.
You don't mention if your structure is kinda like a dynamic "group" or more like a dynamic "queue", or how you plan to "tell" the program what is in the structure. More details on this might lead to a more rounded solution.
Of course XML is just text, and _creating_ (simple) XML is the easy part, so constructing it as a simple string, using StringTheory, might be the easiest way to go.
cheers
Bruce