You can add another extent to a storage area, but the current variable extent will become fixed at its current size. If that's not desirable, then you will want to shut down and remove the (presumably small) variable extent and then add the new ones.
You may also want to ask yourself whether you want to or need to take this approach. Do you really need several fixed-length extents in this storage area? You didn't mention your Progress version or license. Are you on a Workgroup database license where you are forced to have several extents to hold multiple gigabytes of data in a single area? If you are on Enterprise, you may want to consider going with an entirely variable structure (and a recent Progress release and OS/file system); it could make your life easier.