Hi,
for the DublinCore namespace the XMP specification actually defines the format for all the properties. For example creator is already defined as an array being able to hold multiple creators.
For those properties that are not defined to contain several values, but you have multiple ones to store, you should think about if they are really the place you want to use for your values.
Because there is usually a good reason why they have been defined the way they are. Maybe properties in other namespaces are more appropriate.
If you still want to go with the DC namespace, you can write the properties using some other type as defined in the spec, of course, but then you have to keep in mind that other applications will not be able to use your information, because it is not following the standardized way.
Regards
Jörg