A comma or tab seperately text file should be fairly open to most program's. As for import, you would just need to have an example format for people to follow, but I understand this might be more difficult for validation of the data.

Is it possible to add the ability to import and export training pl;anner entries, for example in CSV, excel formats, so we can import entire training plans or export to our own calendars in mail clients (outlook, google/yahoo calendars)?

It would be really useful to have my training planner entries on my phone/laptop complete with reminders etc for example, especially when travelling, and to add training plans from elsewhere.