Teamup supports the import of events from other calendar applications, spreadsheets, databases, and text files. You can use iCalendar or CSV files.
When using .csv format, you can specify multiple sub-calendar names for each event. When the events are imported, they will be assigned to all specified sub-calendars.
In the .csv file, add a column named 'Calendar Name' (case sensitive). List the sub-calendars for each event in that column. Separate each sub-calendar with the | character:
See here for how to import into Teamup Calendar in this way.