Underfill API?

Good Afternoon ~

We have several employees that are hired as an underfill which means they work at a different grade that what there positions is set up with. Meaning the grade they are assigned on the assignment screen is not a valid grade on the job or position. 

This is accomplished on the front end by answering some decisions that are not part of the update_us_emp_asg API.

We are in the process of automating our on-boarding processes so I am curious to know of there is an API that will process this without error. 

Thanks much!

Gwen