this could be a solution :
you can realize all you mapping with the BizTalk Mapper, then you right-clic on the map and choose validate to access the generated xsl and extension object.
Then you can modify you xsl to use the result of you some functoid for the other.
I’m not sure you can realize what you want with the Mapper and there may be some tips but i’m not sure it will be keep if you decide to change one thing in the map (because the mapper generatevariable name)