{{#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. 3, Op. 60 |Piano Concerto No. 3 "Ballade", Op. 60 |Concerto | |Op. 60 | |3 | | |Romantic |1940 - 43 | |1951 }}