You may not use the case. By the way Optiplex 780 is the BTX format , but Optiplex 9010 don't look like the BTX format, because the pcie slot is in the right hand side of the rear panel connectors, which is the typical ATX motherboard layout. For BTX the pcie slot is in the left hand side. Like you said correct me if I'm wrong.
Even the 9010 Mb may fit into the Optiplex 780 case, but you may find the screw holes may not line up the 780 case. Also when you look at the back I/O panel, and the MB pcie slot layout, they are not same too.
Optiplex 9010 SFF MB
http://www.txcesssurplus.com/servlet/the-14289/Dell-F3KHR-Motherboard-for/Detail
Optiplex 9010 SFF case
http://www.pcworld.com/product/1233614/optiplex-9010-all-in-one-desktop.html
Optiplex 780 SFF MB
http://stuartconnections.com/products/Computers/Components/Boards/Optiplex_780SFF_3NVJ6/DSC02654w.jpg
Optiplex 780 SFF case http://www.reusetek.com/dell-optiplex-780-small-form-factor-desktop-computer/
So if you can take off the back I/O shield and don't use it ( but it is not good ), then you can, but I recommend keep the cpu, ram, and other parts, MB and get case to build the PC. Call or email Dell to make sure what format it ( Optiplex 9010 MB) is. If it is the normal any ATX format, then get the mATX case.