MILAN, Dec 13 (Reuters) - Banco BPM (BAMI.MI) has always liked the idea of joining forces with a similar-size rival to create a third large banking group in Italy but is open to assessing all options, Chief Executive Giuseppe Castagna said on Monday. Speaking at the annual conference of banking union FABI, Castagna said Banco BPM was "on the market, if anyone is interested they only need to come forward". Asked about comments earlier on Monday by UniCredit CEO Andrea Orcel about a potential tie-up with Banco BPM, Castagna said every bank was compelled to examine whether any merger option added higher value compared with stand-alone plans. When asked about Banco BPM, Orcel had earlier told the same conference "everything is possible and nothing is possible".