In practice, what you need is a script which uses the very same event command as the default shop and just replaces the shop scene?
This is actually possible
However, if you mean a script which could modify the way items are added to the shop via event... well, this is more complicated.
To be honest, you could make a script which receives parameters via Script Call, but this would mean the shop event command would became useless.
So, what do you exactly need?
Forgive my question, but I haven't fully understood your request...