{{#imslpmsg:addfile}} Use a pipe after addfile to separate, then put the number by which you want to increment the file number displayed; for example, {{Addfile|1}} would increase the file number displayed by 1 (i.e. one cross-referenced file in addition to the files that are there).
{{#imslpworkentry: Piano Concerto No.2, Op.50 | |Concerto |Instrumental (other) |Op.50 | |3 | |Sergei Rachmaninoff |Romantic |1926/27 | |1928 }}