Here are three ways to achieve parallel compression.
1 - use a compressor with a mix control, set your compression to be as hard and as heavy as you like - don't worry if it sounds totally lifeless and artificial because you're going to use the MIX control to add just enough of this to the original signal. If you don't have a compressor that allows this try the freeware Reaper reacomp from Cockos (see the links page).
2 - create an effects track with your choice of compressor on it, set to stun then create a SEND to it from your audio track. You can mix the crushed audio in with the original as needed. Consider routing both to a GROUP track if channel routing in a DAW gives you pleasure, or else link the channels so that you can adjust the instrument as a unit.
3 - duplicate your audio track, kill-compress the duplicate and mix to taste. Again, route to a GROUP or else LINK channels for ease.
Note - PHASE issues - if your DAW does not compensate properly for plugin delays you will get horrible phase problems between the tracks - either compensate manually (read your manual) or else...... actually, if you need me to tell you how to do this just read the manual or else change your DAW.
Extra Note - FX/Duplicate tracks and MIX controls. In general you will want make the processing on an effects track or on a duplicate track 100% wet - in other words all of signal that runs through the unit is processed. Just occasionally I find that I can get a result by allowing a little of the unaffected signal through (mix at just a little less than 100% wet) that I can-not get by simply mixing the full wet return with the original track. I find this most useful with reverb and delay effects but be aware of it when you try parallel compression.