There is no relative mode for knobs in the Minilab MIDI Control Center.
Sorry i wrote Minifreak. You got it anyway.
I did'nt know Minilab did'nt have Relative settings. But i believe it is relative encoders. Perhaps they work relative anyway. Have you tried it just by setting the application parameters to Relative mode?
If it does'nt work, then i suggest you contact Arturia support.
BTW: I do think all Arturias applications should have Fader modes and such to use with all kind of absolute controls.