Well, you can't start with 12 dozers, I think its 9 or 10 max, but if you want to build them just change the commandset to allow the extra stuff, you will probably need to make it so the commandset can switch between two in the same way as GLA workers can change.
yea...you would need extra slots cause the command center hose some attacks as well... but basically its a copying of all the commandbuttons for all the dozers and stashing em under one or 2 command sets...