Joe,
I can assure you that using the original injection molded article as a master for making molds for resin casting CAN be done - quite easily - if you know how.
I've been doing this type of thing for more than a decade - and if you want to have a look at some of my resin work, see this link to a model I've built.......
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=36907
The model you see is a 1/12 scale model - and it's a resin copy of a spun cast pewter original. I made RTV latex molds of all the parts, cast them in resin, and detailed and assembled the model. Came up pretty well, don't you think?
Now, the topic is really too complicated to do a "how to" in this forum, so email me at the link below and we can start you off moving in the right direction - remember that the materials are fairly expensive, so mistakes can be costly (hear the voice of experience here?) - it's better to get a little info to help you start off right.
Hope to speak to you soon.