Hi Keith and welcome. Quick answer is physically yes but legally no without NFA Registration and $200 Tax Stamp.
The full answer is while physically possible, the change would bring the firearm under the legal overall lengrh and subject it to becoming a short barrel shotgun which the ATF regulates. The design 9f the grip on the Shockwave and similar firearms is how companies are skirting the regulations. Cheers, stay safe and enjoy!

James G is right. Don't do it unless and until you have either the right tax stamp, or you are an FFL class 5 SOT holder!

Hi Keith, yes sir, this will fit the 590 Shockwave, but I'm not sure of the legality of doing so. As long as the Shockwave remains at least 26", you are ok. If putting the pistol grip on results in the Shockwave being less than 26", then you have created a short barrelled shotgun.
Stay safe & have a great day!