Importing is no problem. The sheetindex is the number of the worksheet you are importing. Exporting if I recall correctly is not so simple. You would need to create your own Java action for that. There are some good libraries in Java to do that.
I am not correct the Excel exporter also supports multiple worksheets. See the appstore here: https://appstore.home.mendix.com/link/app/726/Mendix/Excel-exporter
with "now" problem i take it you mean "no problem". Am i correct?