a purchase group is a one or a group of people responsible to create purchase orders, but they usually do not request materials, this is done by requisitioners.
A reservation can be created at any time, but has nothing to do with a goods receipt.
Contrary, if you create a reservation, then you have have the intention to reduce stock, which may lead to a new procurement proposal (requisition).
if you want keep your strange process, then you have to hire an ABAPer to make it possible