Can a rca/xlr converter work in reverse??

Theoretically, it could be possible.  I don't know what's inside the Genelac di8a, but if it's just a conversion transformer, it could theoretically be possible.  However, you would have to modify an XLR cable to have two male ends.  Normally, an XLR cable is directional -- on the cable the female connector (with holes) is always connected to the source and the male connector (with 3 pins) is always connected to the target.  I don't recommend doing this modification because we don't know the circuit or impedance characteristics of the Genelac.  You would probably be better off getting a Jensen Transformer PI-2XR or PO-2XR converter box.

I see that the Genelec DI8A uses active circuitry to accomplish the conversion, so it definitely cannot be used in reverse. You would be connecting two actively driven outputs together and connecting two inputs together, instead of connecting an output to an input. Auxinput’s suggestion of a Jensen transformer is a good one, especially the PI-2XR (or two PI-XR if you are using monoblocks), although they would cost in the vicinity of $275 plus cabling for two channels of conversion.

Regards,
-- Al