Hi,
Create a Bex Query on Source DSO, in which create a calculated/formula key figure and set "Median" in formula properties -> calculate results as. Use the same Bex Query as source to feed target DSO in APD.
Please refer the below link for more info:
Direct DSO:
http://help.sap.com/saphelp_nw70/helpdata/en/c0/99663b3e916a78e10000000a11402f/content.htm
Calculate results as:
http://help.sap.com/saphelp_nw70/helpdata/en/17/82853c2dc5c505e10000000a11405a/frameset.htm
Thanks,
Surya