use ADSL modem like a analog to digital converter
The following diagram block diagram taken from the xDSL Modem/DSLAM page at Texas Instruments  illustrates what you'll find in a typical ADSL modem:



So while your general assumption that they contain an ADC and can interface to a PC via USB or Ethernet is correct a few likely challenges are:


There is some coupling and filtering on the analog side of things. That may be possible to bypass if you know what you're looking for but even that can be difficult with multilayer PCB designs.
The ADC and CPU may well be either proprietary ASICs or otherwise require an NDA to get access to the datasheets. Even if you can find that information tracing how everything is connected together is likely to be very difficult.


In general while not impossible it's likely to be far easier to either purchase or design something for this purpose. You'd really need to state your requirements but some fairly easy to use data acquistion solutions at the lower end of the spectrum are only $20 or so.