Stereo or DD5.1?

There should be a way to do it. Maybe this link will help.

I expect that you will probably have to decode the existing AC3 track to its 6 component channels and import them separately; then, on rendering, output a new AC3 track (probably require the purchase of an AC3 plugin) or export the channels as mono WAVs and use an external encoder (e.g. Aften).

5.1 is easy in Vegas.  just split the 5.1 ac3 into 6 wav files (BeSweet or Cinematize).  Set your Vegas file to 5.1 and add each of the tracks in the appropriate layer.  You can also adjust the amount of separation you want on the layer.

When starting a new project in premiere, go to audio, and under master track select 5.1.  Then open with six mono tracks and no stereo or 5.1 tracks.  When the project is opened, label each of the six mono tracks with the appropriate labels: c, l, r, sl, sr, lfe.  Go to "audio mixer" under the "window" tab.  There you will see your 6 mono tracks, each with its own 5.1 panning control, with a little dot indicating where in the mix the mono channel is.  They all start in the center.  Drag each dot for each mono track into its appropriate slot.  The LFE you put in the middle.  Separate the decoded .ac3 from your DVD and import each .wav separately.  Drop the appropriate channel into the appropriate track.  When exporting, go to "export audio" and encode the file as an uncompressed 5.1 wav.  You then can take the 5.1 wav into whatever program you like for .ac3 (like Besweet or AC3Encoder) and finish it off there.