well, yes and no.
I think a good way to explain this is through the 4 level protocol model. We have the application layer, the transport layer, the network layer and the link layer.
Each layer can be swapped out for another one - in this case, your Application would be (for example) HTTP, transported over TCP, over an IP network, over a physical link which in turn communicates using ADSL2+ and PPP over ADSL (into which IP is encapsulated).
The software layers can be done quite easily, the hardware layers less so
The physical link would have to be electrically compatible with your network and so on. To turn your pc into a modem, you'd need to.. well build a modem that 'talked' ADSL 2+ and be compatible with the current network(so be compliant with that standard. And... lets just say your phone company will be less than thrilled, especially if you accidentally introduce a lot of noise into the network or worse).It would probably be easier to buy a modem, rather than source all the parts, build all the boards and code the firmware for the modem.
You could however use your PC as a router as suggested in the comments to your question.