Hi,
thru any other data sources or master data loads have you loaded overtime data to bw side? if yes then think about look ups. just add related info object your cube(which feeds from 0hr_py_1) and load data thru look up.
Consider below suggestion as last option:
If Overtime field very important to add your data source then create z transparent table and load data thru abap program to ztable. from ztable you can do enhancement.Ztable always need to fill before trigger your data source extraction.
Thanks