You can do it a couple of ways, none of which are perfect.
1. If you want a little string solo--for one solo instrument, just insert the solo string in the score (shift/I).
2. If you want a divisi solo string line, you could add as many solo strings as needed to the score. Each solo string is obviously one player, so if you wanted 4 celli, you'd have to make 4 solo celli parts in the score. Not perfect, especially if you have a lot of solo divisi parts.