Michael, they have buffer tube adapters so you can add an ar15 pistol grip and ar15 buffer tube to add a brace that way. It is the route I would go. Dont believe you can add a stock legally, so make sure you add a brace. Found several reviews of people using one, its the ergo buffer tube adapter. Here is a link to one available on buds that will fit your 590 shockwave if interested. Hope this helps, link here.... budsgunshop.com/product_info.php/products_id/714005305/ergo+moss+500+tact+stk+adptr+black

Michael, they exist but are extremely popular and therefore hard to find in stock : https://gun.deals/search/apachesolr_search/699618782622 --------------- Another option is here : cdarmssupply.com/Mossberg-Shockwave-Adjustable-Brace-System-Featuring-The-SB-Tactical-SBA3-Brace--Black-Aces-Tactical_p_424.html