IN CALLING last June for greater contacts between Soviet and American citizens, President Reagan voiced an "impossible dream" of a time when people of the United States and the Soviet Union could travel back and forth freely. It turns out that the Kremlin also has an "impossible dream" in respect to exchanges, one as characteristically Soviet as Mr. Reagan's is characteristically American. Moscow has just voiced its dream in negotiations to write a new exchange agreement. It is to enlist the American government as an auxiliary police force to send home Soviet citizens who come to the United States on an exchange program and want to defect.

The basis of Soviet concern is not hard to understand. To gain prestige and hard currency, Moscow has sent abroad the most brilliant stars in its culture, only to find a seemingly endless succession of them deciding to enjoy the freedoms and favors of the West permanently. It's been a tremendous embarrassment, and it must be doubly painful for the Soviet Union to come now to the American government (not for the first time) and acknowledge that the wicked capitalist state is not merely the source of Moscow's distress but also the single source of its ultimate possible relief. For while some defectors, such as Stalin's daughter, can be lured home, most artists, it seems, cannot.

One could speculate on how the Soviets ever got the zany idea that Ronald Reagan might be recruited to moonlight as an enforcer for the KGB. It's more to the point to indicate other possibilities open to Moscow. On the level of the cultural superstars, if it relaxed a bit and granted that cultural folk tend to the footloose and the cosmopolitan, it could permit a more natural back-and-forth that would do much to moot the question of defection.

On the more basic level of free travel, free emigration and the rights promised its citizens by its laws, the Kremlin simply needs to be reminded, again and again, of the inhumanity of its policies. Why is Yelena Bonner not permitted to go abroad? What about Anatoly Koryagin, the psychiatrist who protested against the use of psychiatry as a tool against dissent and who is reportedly in desperate condition in Chistopol prison? To Moscow's arrogant request to have Americans police Soviet exchanges, Washington needs to reply by asking how it is possible to have a decent program when official cruelties continue to be inflicted upon people such as these.