Will I be able to get the previous ESPN season history later?

There isn't a way to do a secondary import to an existing league unfortunately. Commissioners could initiate a new import of the league in order to make the full history accessible on Fleaflicker. Then they could either transition owners over to the newly imported league and delete the previous league, or they could opt to keep both and label the additional league as "Historical" so that it's always there for reference. Another option is to retain the league's history by taking screenshots of the past standings and then uploading them to a tool like Google Drive or Dropbox. The resulting file(s) can be shared with league members and then linked to on the Fleaflicker league forum for future access.