Drill a hole in a flat spot of the detail piece, just be careful not to drill into the helmet layer. Then mix up some resin and pour it into the hole until the detail piece is full. once the resin cures, use bondo to make the hole disappear.
Also, (and I learned this the hard way) with bondo, less is better. You don't want to cover your whole piece at once, instead work in small areas. Too much bondo covers up all the details that you painstakingly worked on in the pep stage to create, and then you have to sand forever to get them back. My advice is to start small. You can always add more bondo if you need it, but once you put too much on, you'll spend alot of tome sanding to get it nice again. When I bondo, I start shaping it with a small square of sixty grit sandpaper while it is still only semi-hard, kind of like cheese. The semi-hard bondo will get a fuzzy surface while sanding, and quickly clog your sandpaper, but it will save you a ton of time. Once it is fully cured, use a clean piece of 60 grit for your final rough shaping, then sand with 100, followed by 180 or 220 grit. You don't need to use anything finer until you start your finish coat of primer.
You may also want to check out this thread for adding details.
http://www.405th.com/showthread.php/19173-Using-Foamies-&-Bondo-For-Detailing
Good luck with your build!