There isn't room in the Powermac case for a second full sized drive. Some people have added them but it requires cutting some metal structure of the case, needless to say, that would void a warranty and metal shavings aren't compatible with computer components. I wouldn't do this if I were you.
The other option is to buy an external DVD drive. Probably the cheapest route would be to buy a Firewire case, and then a DVD-ROM drive. The drive will be cheap, but the Firewire case will cost you.
Everytime I think about this I get so pissed off at Apple for not putting a second full sized drive bay in their towers. WTF, these towers cost 2k-4k, and they don't even provide space for adding a second removable media drive? WTF!!! This winds up costing Mac customers a good $200 extra when they try to add a drive. When I added my CDRW drive, Firewire drives cost around $150+ extra, compared to internal drives.
Why Apple did this I have no clue...well, no I do have a clue. Apple has a patent on Firewire, and I think they make royalities off of every Firewire drive purchase (I might be wrong, can anyone confirm this?). So it would be in Apple's interest to force Mac owners to buy Firewire devices over ide drives.
I hate to think that Apple would be so slimey, but they are a business and when the Powermac case was introduced Apple was in the gutter and desperate for money. I wouldn't be surprised if the reason we get screwed when expanding the "expandable" tower is because of Apple's greed.