Courtesy Aaron and Stan:
To install an x86 additional driver on a 64 bit version of Server 2008 , you first need a running x86 Vista OR Windows 7 system.
1) Install the x64 printer driver on the server.
2) On your x86 PC, download and extract the x86 printer driver to C:tempprinter_name.
3) On the PC, locate the WindowsSystem32DriverStoreFileRepositoryntprint.inf_xxxxx directory and copy all files and sub-directories from ntprint.inf_xxxxx into C:tempprinter_name.
4) Copy the printer_name folder to the server.
5) Add an additional driver to the printer, use the printer_name folder as the source for the driver.
If the x64 host asks for the NTPRINT.INF on the Windows Media during the installation of the additional driver, simply browse to the printer_name folder and complete the installation.
source: Technet Forums