Another very easy way:

1. Get a non-Cingular BB (from a friend or family?)
2. Install YiM on it
3. Use "Switch Device" on Desktop Manager to move your service to this non-Cingular BB, making sure you also transfer application data by checking the appropriate checkbox
4. Switch back to your Cingular BB (follow same steps as 3)
5. The "BBFS2 [YHO]" Service book is now copied on your Cingular BB
6. YiM works perfectly on your Cingular BB!! IM away