Yes, the possessive form of a proper noun can act as an adjective.
The proper adjective is "American," derived from the proper noun "America."
"Chicago" can be a noun (referring to a city) or an adjective (describing something related to the city).
The proper adjective for southwest is "southwestern."
The proper adjective is Haitian.
The proper adjective for Portuguese is "Portuguese."
Vietnamese is the proper adjective for Vietnam.
The proper adjective for George is Georgian.
Mayan is itself a proper adjective.
The proper adjective for Siam is Siamese.
The proper adjective for Georgia is Georgian.
Freudian is the proper adjective for Freud.