If I have 2 bookshelf speakers and an amp with no sub output, can I still add a sub to make it a 2.1 setup?
You can get a subwoofer with a high level input, such as this.
If you've only got one speaker channel, you'll have to get a splitter/switch to power both from the one amp.
Whist its not a purist way to go I have definately done this and fed a non-sub feed to a sub unit and just tweaked around to get the right balance.
You have two options depending if its an active or passive sub unit (does it have a built-in amp or just terminals to connect to the speaker out on an amp?)
Im lucky as I have a powered sub and run one of the phono connectors to it from my usb DAC and the sub naturally filters out the mid/high frequencies and amplifies the bass signal.
As bass is providing just a low frequency fill you dont need to worry about pure stereo it tends to be an omnidirectional sound so can be driven by any source (left or right)
If you are just connecting it via speaker cables then you could piggy back one of the L/R speaker out terminals - just don't join them up across the L/R. If you have a second group of speaker connectors you can use this too just make sure you have both groups selected on the amp.
Good luck on the connect up.