This is possible with the newer API introduced in SDL Tridion 2011, but I don't think you can do it in 2009.
From the SDL Tridion 2009 SP1 Content Management Implementation Manual
(Section 9.15.3, Using a Custom URL to fill in the value of a field)
Multivalued fields — Should use an array for the returnValue. Each entry in the array represents a value to insert in the field.