Sorry for the delayed answer.

1) Can we integrate the powershell scripts to the Policy Management or shall I say can we add this script using the GUI in policy management?

[Answer]Per my knowledge, there is no option to integrate the powershell scripts in the PBM because the facets are predefined and can not be modified.

2)How can we implement a policy in powershell script that upon creation of a certain store procedure which does not conforms to the performance best practices it will autmatically returned error and it shall not be created?

[Answer]If I understood your question correctly, the answer is no, the policies can evaluate the created objects and defined facets.