Your mixer is plugged into your computer right?
Given that, this is what I would do:
(I understand this is going to add a bit to the process, but desiring to add the vocals, and proper production techniques really do point to this solution... then again I'm not an official pro.
)
Plug your EX7's audio out into your mixer.
Bring up a song you've got.
Mute all channels on the EX7 except for one channel.
Play the song, make sure the levels on the mixer aren't clipping (getting up to 0db).
Open up Sound Forge or whichever audio program you have that will record audio from your soundcard's Line In.
Check the levels again. Make sure that at no point is the audio clipping.
Hit record on Sound Forge (Or equivelant) and play on the EX7.
Save the resulting wav.
This will allow all the sounds from the EX7 to be used.
Repeat this for each channel that's used for the song.
Record the vocals.
You can then open up Acid or any other multitracking program you have.
Import the wavs you've recorded from
the EX7 into the program.
Put each one on it's own track
This part sucks, but you'll want to make sure that each one starts at the right point so they're all synced (Anyone with a suggestion on how to do this most efficiently?)
You can then use the mixing functions of the program (Acid) to set the levels for each track individually and even create changes over time.
Export the project as a mixed wav file
Open soundforge or same, normalize the song. Using
a dynamics compressor if available, add suitable compression and normalize again.
Burn to CD.
If I've missed something or you'd like more detail, feel free.