Question: Is it possible to merge two or more fields just for import in order to have at least 15ha to import?
You can import multiple field surveys into one osd file to get the total area above 15ha (eg 6 + 8 + 8 = 22ha). Then define the boundary to design one field (say 6ha) and then export. You will then be asked to pay for the total survey area (22ha). Proceed and pay for the full area. Save the osd file, then 'Save As' another osd filename and then adjust the boundary to another field (next 8ha) and design.
When you export again you will not be asked to pay again. Note the fields should be within a few miles of each other so the OSD file is not too spread out. Go to Tools > Import then select the survey file. Import each of the survey files to the same osd file creating one combined field.