I'm not sure if you're referring to exporting down to user-specified level from MindManager to Excel, or going the other way from Excel to MindManager.
The problem with the first is that MindManager, strangely, does not include an option to select topics by level. However, the MAP add-in includes this facility, and you can use it to add levels cumulatively.
To export to Excel you select the levels you want using the MAP add-in, then click on Task Filters. Check that Hide Non-Matching Topics is selected, and click on Current. All but the topics at the level(s) you selected should disappear. If you now export to Excel, only the selected topics should appear in the spreadsheet.
Going the other way is a little trickier and I haven't found an easy way to do this yet. I have found a way to do something like it using a pivot tables rather than grouped data, but there are some issues with this approach.