Why don't you load the data as periodic to HFM and use native functionality to return YTD.
From a pure solution perspective. You could create a YTD scenario and have a business rule run to populate the YTD scenario.
If this is actual data you may want to revisit this process, it would not be typical to load actual to pbcs prior to hfm. If HFM is consolidating YTD it is typically simple enough and recommended to get YTD data from the source ERP system.
Hope this helps,