Adding space to a table fragmented with round robin

I have a table of 20GB (of data - the indexes are detached), fragmented over 5 dbspaces which are all about to run out of space. We're on 7.3. Physical disk space is at a serious premium so I'd be reluctant to add a whole 2GB chunk to each dbspace. If I added chunks in smaller increments it would mess up their nice neat topography and I've no idea how long a given amount of space would last. (The system will only be running for another 3 months and consequently they have a budget for more disks of zero). Ideally I would like to simply add another 4GB fragment but I don't believe this is possible with round robin - is it? What strategy would the panel adopt? Thanks, Andy Kent