Can we assign multiple Grade in a single Lot or How To Assign Multiple Grades to a Single Lot Number on Inventory Transactions

This requirement is not supported.
Per the design of the e-business suite, a lot has one grade value only. If there are multiple grades then it is different lots.

See for instance Oracle Inventory User's Guide Release 12.1 Part No. E13450-02, CH 12 On-hand and Availability, Item Reservations. The following is said:
"A lot can have only one grade at any given
moment, and grades can change over time."



Similarly, go to Item Lots / Maintain Lot Numbers form (INVTDILT):
Inventory: (N) On-hand, Availability : Lots
This form is for creating, maintaining or just reviewing lots.
See there is a field "Grade" and there is no way to assign nor display multiple grade values for same lot.