I typically call "tender" springs the ones with 0 spring rate. (only meant to hold a spring in the seat). A helper spring can be bought in multiple rates.
Hope those help. As you can see the tender does not have a shaft to slide over, so you have make sure there is a seat in there for the collar on the tender. Worst case you will oval out the tender from the slight side loading of the spring under large oscillations.
Zack