Apple Mail does not import the OLM format files. Apple mail can only import in MBOX format. Thus all you need to to do at this moment is- convert the OLM files to Apple Mail archive.
There is no freeware solution to this. Only a paid tool can get your mails converted in the right manner. It is important that you take a free trial of the tool before purchasing it. Not every tool is worth the price. Make sure; the tool offers a refund as a guarantee for their success.
The tool I tried on offered me much more at the reasonable tag. Gladwev's OLM Converter Pro is a brilliant tool in all aspects.