Depend on how you shared the connection before. If using RF like most people, have to get a composite-to-RF converter, which almost every supermarket got sell. If using S-Video like one unique case here, have to get a component-to-S-Video converter, which is harder to find. If using three wire composite(yellow video + red and white sound) then it will still work without needing additional devices. The decoder can output in HD and SD at the same time (when watching HD channel, HDMI will output HD while composite and component will output downsampled SD simultaneously). But cannot output 5.1 and 2.0 sound at the same time. If you enable 5.1 sound then only coaxial digital will be enabled, all other outputs will become disabled. On 2.0 all outputs are enabled. So if you use splitting you need to improvise (i.e. turn on 5.1 manually when watching movie and remember to turn it back off when finished).

You can do yourself. First you need to find out how the split is done at the moment (hopefully it's RF or composite. S-Video difficult a bit because the converter very expensive online and is almost impossible to find in shops). Astro person can tell you how it's currently split. After that, you just need to get the required device and set it up. If in doubt, get Astro person, then get the contractor who wired your splitting for you.