Custom units would be a useful feature to have.

I'd probably implement this by having some kind of API to create an instance of a Converter class which lets you register custom units at runtime somehow. With the current way Convert is written this would be a bit of a hassle to add since all the current code uses the one big lookup table for conversion ratios.

I'm pretty busy these days so unfortunately I can't make any guarantees on when I'll have time to add this feature.

Any update on this?

I ended up saving custom units in DB as I had such requirement.
type CustomUnit = { name: string; conversionFactor: number; baseUnit: string; measurementType: string }

So if a bag is 50kg
My custom unit looks like:
{ name: 'bag', conversionFactor: 50, baseUnit: 'kg', measurementType: 'mass' }

to convert to grams, I can do:
convert(customUnit.conversionFactor * value, customUnit.baseUnit).to(targetUnit)