Adding additional admin fee on top of civicontribute membership signup
If you haven't tried using priceset then i would recommend you to use priceset 

The best way to have membership fee and administration fee would be using a price set which allows you to have multiple option 

reference: 
https://wiki.civicrm.org/confluence/display/CRMDOC/Membership+Price+Sets

https://docs.civicrm.org/user/en/4.6/membership/membership-price-sets/

I guess this helps !!!