Copy subgrid records of one entity to subgrid of another entity

Hi

Need inputs on the below mentioned functionality for writing a plugin

* I have entities A & B

* Lookup field of B is present in A entity

*Also, there is a subgrid on form of  entity B

*Need to copy the records from the subgrid of entity B to subgrid of entity A based on the lookup filed of entity A. Hence using lookup id how can i move forward to achieve this functionality??

Thanks in advance