{{#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: Songs and Dances, Op.33 |Lieder und Tänze - Zwanzig kleine Klavierstücke, Op.33 |Piano piece |Instrumental (chamber) |Op.33 | |20 | | |Romantic | | | }}