the easiest solution is a "Y adapter" to split the outputs. Otherwise you could have a tech install another set of outputs for a sub, but depending on how they do it the yield will be the same as the adapter.
I take it your sub doesn't have a cross-over such that you can connect your sub to the pre-outs, and then connect your amp to the outputs on the sub?
Another option that is best IMHO, is assuming your sub has high level inputs, connect it to your power amp via the high level inputs. This is how I normally prefer to do it, except when it's being used in a HT setup where you have LFE's to worry about.
