a couple questions to make sure we understand what you need:
Which language do you use to develop your desktop app ?
Do you need help with the OAuth process too ?
The process I'm hoping for would be the following:
1) A webpage (ideally PHP) that a user would visit, it would do all the calls to load there vital league setup information (league size, team names, logos,etc.).
After processing, the user could then save a file (XML) to there computer. Then I'd take care of all of the processing of that file with the current desktop application.
2) Another page a user would visit where the user could then upload a file (XML). This file would be essentially a roster list (of players they drafted using my desktop application) that had all the vital information, and it would essentially update their league's rosters on Yahoo.