I'm looking at trying to do the same exact thing. It would be nice to get a real answer because I just got started with limesurvey. This might work, but here's what I've found out...
1) Import users into mysql lime_tokens_<sid> (sid=survey id).
2) Fill in random information into the 'token' column. Make sure it's random.
3) Query the db for the user, email, and token, and formulate the URL using that information.
4) You can then write some code to generate an email outside of limesurvey.
It would be great if others can comment if there's an easier way to do this. Thought i'd share what I found. Here's also a ticket relating to this.