Is a Helmert transform necessary to convert between WGS84 and NAD83?
Short answer is yes. You need to convert from your coordinates' native reference frame to the NAD83 realization's reference frame, then possibly add an adjustment. In the case of NAD83(CSRS), which I use, there are 3 steps:
- Native RF -> ITRF96(1997.0)
- ITRF96 (1997.0) -> NAD83(CSRS)
- Grid shift
Steps 1 and 2 require the Helmert transformation. Step 3 uses interpolated shifts from a raster to apply adjustments due to tectonic motion and isostatic rebound.
NAD83 is intended to be locked to the North American plate so its realization must rotate with respect to the reference frame. In CSRS this accounted for partly by the final reference frame transformation and partly by the grid shift. (It's fairly difficult to find complete information on this online -- when I was writing software to transform point clouds, I had some generous help from the NRCAN scientist who's responsible for it.)
The IERS publishes transformation parameters for going between various ITRF realizations: here.
I got the ones for CSRS from The Evolution of NAD83(CSRS) In Canada, by Mike Craymer (2006). This page has the transformation parameters between 2011 and ITRF. (This one has a spreadsheet with many transformations.)