It's not his pal Akeem but an uncredited star in a cameo appearance who really saves Robin Hood in the end. And now, thanks to the magic of the VCR, you can salvage those lemon rental movies by inserting your own cameos from superior films. All you need are a blank tape, two movie cassettes, and two VCRs (with standard cables running from one's ''out'' jack into the other's ''in''). Some examples to get you started: MY DINNER WITH ANDRE Hit lady Anne Parillaud, in La Femme Nikita's restaurant- assassination scene, interrupts Wally Shawn and Andre Gregory.
THE SHELTERING SKY Kit and Port get over their endless desert ennui when Hope and Crosby perform ''Moonlight Becomes You'' from Road to Morocco.
THE DOORS Jack Nicholson, as the grinning acidhead rock musician in 1968's Psych-Out, pops in as a member of Jim Morrison's wacky crew.
THE GODFATHER PART III Al Pacino, in any scene from Scarface, emerges as Michael Corleone's illegitimate, drug-dealing Cuban son.
GHOST The cast of Topper (Cary Grant, Constance Bennett) appears as Patrick Swayze's pals beyond the pale.
