You don't need to use SPX files to do this.

Assign your extend bones, and then make them into spring-bones in 3DX, choosing the settings you want as a starting point. Then send that character to iClone. You can further adjust it there.

Note:
SPX files are bone-specific, so you have to make sure they match the skeleton you're using.

I use daz3d to create character. Then import into 3DXC7 to convert to iCharacter in Iclone. I turned on spring bones. But in the iclone can not import SPX files for the character. Breasts of the character can not be moved. How does the character's body soften and shake as the character moves?

I tried importing icharacter into 3DXC 7, but also can not import spx Soft_Breast or Mild_Soft files. It's an error
 "Failed to load spring setting, and please refer to SPX v1.2 rule.
3DXchange 6 export the new .spx files (SPX 1.2), and you can use the text editor to open it and modify your spx accordingly."