Hi Dan, you can, but it is not so easy. The stock on the Sub-2000 is removable. It also acts as the end of the buffer tube, so if you take it off, you will get access to the recoil spring and the bolt. This is in contrast to the AR-15's tube, which is capped off at the end - the stock just rides it. As a result, the Sub-2000 stock is kept on by a retaining pin that goes straight through the buffer tube - yanking the pin removes part of the back of the stock, which gives access to the buffer tube and lets you pull off the rest of the stock.
Removing the OEM stock will lose one of the big reasons people buy it, which is it folds.
Stay safe & have a great day!

Hi Dan and welcome. Yes, here are some butt stock options : https://unitedfreedomgear.com/collections/keltec-sub-2000