If you are talking about using the source values from one dimension in your source file as the source values for the mappings for multiple target dimensions in FDM then this is standard behaviour, just associate the single source dimension with multiple target dimensions in your import format. However, if you mean mapping a single source value to multiple target values in the same dimension then you will need to setup some logic accounts to help you achieve this. Check out the section on logic accounts in the FDM admin guide.
When you define an import format you add a line for every target dimension you want to map and then associate with a field/dimension from your source. You can have the same field/dimension source associated with more than one target dimension. Then in your mappoing definitions you map using the values from the source dimension you specified. This is all detailed in the the section on Import formats in the FDM admin guide, so its best to use that as a reference. Its very simple functionality to yse and setup.