As you've noticed, K-3D uses SI units internally for consistency. What you may not be aware of is that you can use other units of measure for data entry. So you can type "3ft", "3 ft", or even "3 ftUS" in an edit box, and it will be automatically converted to SI.
What we don't have at the moment is display of arbitrary units, so there's no way to tell the UI that you would prefer to see units displayed as feet. In the preceding example, "3ft" would be converted and displayed as "0.914402 m" after you hit tab or enter.
I've added a feature request to the tracker for this.
Cheers,
Tim