I do this with WaveLab 9 and iZotope RX5 and it works.
For example the registry entry for iZotope RX5:
[HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\Tools Menu\iZotope RX]
"ExePath"="C:\Program Files (x86)\iZotope\RX 5 Audio Editor\win64\iZotope RX 5.exe"
"MenuText"="iZotope RX"
"StatusBarText"="Edit with iZotop RX"
"Type"="WaveEditor"
(after closing regedit restart SONAR to take effect)