With all of that being said, I have some observations based on your posts in the past few days.
You're using BP to interact with a list of FI postings in FBL3N. You have indicated in a separate post that you need to be able to dynamically move through the rows.
“So basically what I want to know is, if I can edit the spied element attribute value to something like /app/con[0]/ses[0]/wnd[0]/usr/lbl[98,$variable$] and set this variable from my process”
This is very possible to do and will work with the interaction method I gave you above; however, if your list of records exceeds what you can display given your screen resolution, SAP will generate a scrollbar. Anything not visible without moving the scrollbar down will be inaccessible to BP interaction. You can “solve” this by scrolling down, but you are introducing a screen resolution requirement into your objects.
As an alternative, consider changing your FBL3N layout in SAP so that it includes Document # and Fiscal Year. You could then change your logic to download the FBL3N data into excel/csv and then into an internal BP collection. From there you could loop into FB03 and for each record pass the Company Code, Document #, and Fiscal Year to get to the posting.
I believe this would save you headaches down the line. It would also better modularize your objects.