Most of the " Easy " solutions are paid, and the trial edition allows you to migrate only few emails, please check the solution listed in the link below.
https://exchangeserverstips.wordpress.com/2013/12/24/steps-to-convert-mbox-file-to-pst-file-manually/
My advice would be "making a copy of your mbox file" and working on it (maybe this solution damages your mbox file)
Best