The speaker level to line level converter is essentially a fancy voltage divider (in the very simplest form you use a couple of resistors).
Going from line to speaker level would require an amplifier - not a big one, but an amplifier all the same.

I believe there are specific adapters available for line level out after market HUs to stock "premium" systems with high level inputs. Browse old stuff in teh Installation forum and I believe you find some posts about these (sorry, no good ideas for search terms...)

/Michael