I understand that. The preferred arrangement is to have everything in consistent units.
However, provided the crew cockpit indications are metric, then the whole process can be figured in metric. This would leave the only area for likely error to be the physical fuel loading (presuming at tank indications are imperial). Providing the crew indicators are metric, then the crew fuel quantity crosschecks provide a final error trap. Not ideal, but I would have thought workable.
Depending on your jurisdiction, you may also have CofA or AOC requirements which would preclude my suggestion.