I recommend you wrap up your existing DLLs into Windows Runtime components. You should rebuild them in Visual Studio 2012.
For 3rd party DLLs, it's required that they do not use APIs which are not available in Windows Store apps. Or else you need to find out alternatives or other solutions.
Please check
API reference for Windows Store apps for more details.
Robin [MSFT]
MSDN Community Support | Feedback to us
Develop and promote your apps in Windows Store
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