no problem to do this with your hardware and the NI-CAN Frame API.
However you have to implement Diag Services and Transport Layer on your own, which is quite a huge task.
I would recommend spending the money for Automotive Diagnostic Command Set which supports
- NI-CAN (PCI, PXI, USB, PCMCIA)
- NI-XNET
- cRIO/985x targets (Pharlap and VxWorks)#
which means if you plan to port your app in the future to a different hardware target or real time target you dont need to change your appl at all.