Sky can convert a single dish, its the same dish only has 2 feeds instead of 1 for sky +. I guess they would just run a second feed from the dish, if your dish has the capacity left (think its upto 8 or 16 feeds from 1 dish with booster/splitter).
If you only have 1 feed, you will not be able to record more than one programme at once, or record one programme whilst watching another. You may, at times, have to set up "dummy" recordings to ensure that your box always uses the LNB1 input for recording. If you look back through the archives you will find more on this subject.