you have to do a bounce past Blender to prep but
go in pose tab set Opensim rig and T-pose
export as DAE set options as showing in https://gyazo.com/b21810c51a89228106404eebf5636884
import into Blender (should be a recent version ie 2.7??)
tweak as needed
SELECT ALL in blender
export back out as DAE set options as showing https://gyazo.com/33929a9d06d901781047d1853bc069ff (there is an opensim rigged preset for this)
upload your textures and then upload your model (please note it will be heavy)