well - voicecorder-vsts do exists, but afaik, noone is free.
..and you btw still have to 'sing' for those too -but afaik still best alternative to a actual vocalist.
You can search for 'voicecorder+vst' on google, but make sure you have a test version and that it does work with lmms, before you pay for it!
In that respect - read the section on vst in the wiki
http://lmms.sourceforge.net/wiki/index. ... _with_VSTs
..You do know about the choir presets in the zynAddSubFX collection -right?
Br.