Recently someone asked if you can use custom XSLT in Biztalk Maps . The answer is of course yes as the map you built .btm gets translated to an xslt anyway if yo look at the code that gets spewed out .
In any case to do that create the map and select source and destination . Click on the map and go to properties. There you will see a property custom XSLT . You can select your custom XSLT here . This feature was there since 2004 but I am not sure if it widely used though , on the flip side using custom xslt you kind of lose the graphical view on the map on what is being done , which many BI analysts like (the visual part of the transformation ) . but sometimes custom XSLT's could be the way to go .