I have control designed for VS2008 and VS2010. Will it work in Power Builder 12 designer as well? or Would I need to create any design.dll
It depends on the control but in general PowerBuilder 12 (and prior versions) can use external controls via OLE without any problem. You add them to your application via Insert --> Control --> OLE menu item, then you'll be prompted with a list of registered object to select from.
When coding you'll need to use the "object reference" or dot notation to reference properties and/or methods. Everything works fine in PB but it's not as user friendly you can't do named arguments and need to get your syntax just (and catch exceptions) right to avoid null object reference crashes.
Hope that I understood your question correctly and this helps.
Regards, Rich