The out of the box solution is to ensure that the column names in your Excel CSV, match identically to the field names that they are being mapped to.
As an example, one of the out-of-the-box fields is called First Name. If your CSV column heading has First Name, then the Import Wizard will automatically select that and match it to the field.
Original question asked from the point of view of EXM. It was updated to reflect List Manager instead.
The Contact import process that you're talking about is actually not EXM specifically, but rather List Manager, which is a sub-product in the larger Sitecore XP. EXM simply integrates with List Manager to provide the functionality which is why it looks like EXM is doing the work.
The CSV Import process can be heavily modified to work with custom Contact facets. For additional information on that, you can browse the following blogs: