So for your case here the blend method would involve trimming away some area like this so there is some space between the pieces:



With this kind of empty space opened up between pieces you can then select edges like this:



And use Construct > Blend to build a smooth connecting surface between those like this:



However in your case here the quality of the base body piece is not too good and that will cause problems, specifically this edge here is actually a creased edge:



You can see the lack of smoothness a bit more clearly in a shaded only type view (here when doing a polygon export with "metallic lighting" style enabled):



So I'd recommend reconstructing that body piece to a higher quality smooth piece first before trying to do this type of blending - the problem is that the blends will not connect to each other well when they're built off of things that are creased to one another.

I'm not sure exactly how the body was constructed, but if it was done with sweep probably instead of only sweeping a half piece and mirroring that you should instead try to sweep the whole thing as one large piece, see here for some explanation of that:

http://moi3d.com/forum/index.php?webtag=MOI&msg=1398.18
http://moi3d.com/forum/index.php?webtag=MOI&msg=1398.19

- Michael