How to transform multiple "old" C# business objects layer to be exposed in WCF

Question

I know how to create [DataContract] to expose my business objects layer, it's a one by one process and, as I have many hundreds objects to process, my question is simple: is there's a "proper way" to expose already created business layer to WCF?

I need to keep my real 3-tiers intact and of course my business objects layer are used somewhere else without WCF so I need to keep it as is if possible...

Here's my BOL screenshot representing the solution structure

Thank you