A process for preparing a bitumen-polymer composition includes mixing and heating a base bitumen-polymer with at least: one compound of general formula (I): R1—(COOH)z where R1 is a linear or branched, saturated or unsaturated, hydrocarbon chain, including 4 to 68 carbon atoms, and z is an integer from 1 to 4, and a compound (II) selected from the compounds able to react according to an acid-base reaction, an amidation reaction or an esterification reaction, with the carboxylic acid group of the additive (I). Also set forth is the composition obtained by the method and a kit composed of a bitumen-polymer composition including an additive (I), in the form of blocks, and a capsule comprising a compound (II).