It depends on what "custom drivers" you have, and maybe on what type of instrument you have.
If your driver is a dll or other shared library, use "Call Library Function".
If your driver is not a library, but just code written in some other language, use the "Code Interface Node".
If your instrument is a GPIB device, or uses some other sort of serial protocol, there should be some documentation with the device that gives the command set. Figure out what commands you need and rewrite them with LabVIEW VISA vis.
The easiest thing to do is contact the company that made the instrument and ask them if they have LV drivers. If they do, great. If not, let them know they should be developing them...