Hello Safaa,
Extreme Policy is a feature based on Enterasys portfolio capability. Only newer devices (X440-G2, X450-G2, X460-G2, X465, X590, X620, X670-G2, X690, X870, but also X770 which is a 'first generation model') support Extreme Policy. And ex-Enterasys devices of course, both switches and wireless (ExtremeWireless, formerly IdentiFi).
If you wanted to get policy feature on Juniper or other third party, it would have to support such feature first. However, there is no straightworward option for out-of-the-box translation of a policy domain to 3rd party devices.
What you can do, is to check the RADIUS attributes that Juniper can take during user/device authentication and authorization, like VLAN (RFC3580) but maybe other things as well (1st gen EXOS devices were based on issuing scripts - called with RADIUS attribute by their names - to reconfigure port on which a user got authenticated at the moment; Cisco Catalysts support dynamic ACL to apply on a port, etc.). These things can be ordered from Extreme Access Control that handles the authentication being an AAA server from the switches' perspective.
Hope that helps,
Tomasz