Interestingly, Kaplan's wording here directly reflects what the AAMC says, which is that test-takers are required to "use metric units, including converting units within the metric system and between metric and English units (conversion factors will be provided when needed)."
So I wouldn't worry too much about miles to feet and so on, and even something like eV to joules will be given to you (there was actually a controversy back in 2015-2016 or so when the AAMC omitted something very similar from a practice test, but then re-added it after an outcry and lots of confusion). What you should absolutely get good at is converting back and forth between different scales in the metric system, using scientific notation. As a quick diagnostic, how good would you feel about expressing the wavelengths of visible light in micrometers? Or in standardized scientific notation (i.e., __ x 10^___ m), or even picometers just for laughs? The ability to do conversions like that, especially using scientific notation, can really pay off.
[Also, as a quick edit -- I do recommend knowing basic temperature conversions and things like that. But your MCAT books/materials will generally give you a good sense of the limited amount of units you should know.]