Will it able to use 32 bit unmanaged library from 64 bit managed application? If yes, how can it be done?
No this is not possible. 64 bit processes can only load 64 bit modules. 32 bit processes can only load 32 bit modules.
The library you link to ships as source and is covered by the LGPL license. So long as you adhere to that license you can compile the code into a 64 bit DLL and use that from your 64 bit process.