(Recasts, adds detail) MILAN, Jan 25 (Reuters) - Italian energy group Eni has agreed to buy Spanish energy retailer Aldro Energia as part of its plans to increase its portfolio of retail power and gas customers in Europe. The group said on Monday its gas and power unit had bought the whole of Aldro Energía Y Soluciones SLU from Grupo Pitma to get a foothold in the Iberian market. The deal includes the acquisition of backoffice company Instalaciones Martìnez Dìaz. “The company aims to increase its portfolio, reaching 11 million customers in 2023 to offer not only gas and electricity but the entire range of services related to home and energy,” the chief executive of Eni Gas e Luce unit, Alberto Chiarini, said. Eni, which like other oil and gas groups is spending heavily on the energy transition, has placed retailing at the heart of operations and aims to have more than 20 million customers by 2050. Aldro Energía supplies energy to 250,000 customers mainly in Spain and Portugal, with a focus on small and medium-sized enterprises. Eni already operates on the retail market in France, Greece and Slovenia. Banco Santander advised Grupo Pitma while Mediobanca advised Eni.