How to reference .dll on a different .NET framework in VS 2010

I work on a product that is running on .NET 2.0. Yes, this is horrible and outdated, I know, but several of our clients have enough trouble just configuring firewalls to connect to our servers that it'd be chaos to change this. I happen to know that several of them are still running XP.

I'm running into problems when I try to convert the product to VS 2010 because the reference to a .dll compiled with a target framework of 3.5 is no longer valid.

Changing the target framework of the dll compiled with .NET 3.5 to .NET 2.0 would be an immense amount of work, because the source would have to be almost entirely rewritten.

Is there any way to add a reference to a dll compiled with .NET 3.5 in the project with a target of .NET 2.0? Again, whether correct or not, the project in VS 2005 is able to reference this without any problems.