{{#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: Tri skazki Op.9 | |Piano piece |Instrumental (chamber) |Op.9 | |3 | | |Romantic |1904-5 |Piano solo |1905 }}