Yes. I kept my original SBLive in when I bought my Echo Mia. I used the SBLive as a hardware MIDI synth/soundfont machine.
You do have to physically route the output of the SBLive to the input of the 24-bit card (for example, you have to run the line-out of the SBLive to the tape-ins or a channel on your mixer.... no, you cannot do it all internally in the computer.) Then just choose the audio inputs and outputs of your sequencer to the 24-bit card and they MIDI i/o to the SBLive.
But I found that I got BETTER sound quality by dumping the SBLive and going with a soundfont VSTi (I use Bismark's BS-16, but rgcaudio.com 's "sfz" is free or you could use any sampler like Kontact, Halion, VSampler, etc.) So I eventually dumped the SBLive.