The upcoming version of NI-FGEN, version 2.2.1, will support selected signal generators, including the PXI-5404, under LabVIEW Real-Time 7.1. You will then be able to program the PXI-5404 in LabVIEW Real-Time using the regular (non-Express) NI-FGEN VIs. The new NI-FGEN version should be available on the web site before long; we'll post a new comment to keep you updated.
Vince Shouba
NI software developer