Relative adjustments to Lightroom images are only available through the Quick Develop panel of the Library module. Lightroom presets use Develop module settings and overwrite values rather than increment them.
If memory serves there was a plugin called Gazoo that did relative adjustments. I have never used it.
That was my experience too. A relative adjustment would be nice when needing to apply the same adjustment to a batch of images without "overwriting" previous edits. I'll take a look at Gazoo and see what it does.
Gazoo will certainly do relative adjustments (and presets..), but if your aim is to make adjustments just for the sake of an exported copy (e.g. for print), then consider Exportant instead - it can make Lr adjustments that impact the exported file only (not the source file):
PS - both plugins are free, and written by me.