Objective C - design advice: Class to store a bodyweight value

No, don't not use NSNumber at all, do not even add a category to it - this class (cluster) if designed for when you need to store a primitive type as an object and little else.

It you wish to encapsulate a weight write a class to do it, something along the lines of (code typed at terminal):

@interface Weight : NSObject

@property double kilos:
@property double pounds;
// etc

@end

@implementation Weight
{
   double value; // stored in a suitable unit, kg, lb, oz, g, etc.
}

// implement getters and setters converting between unit of property and unit of value

// implement dependent property methods to setting, say, pounds produces a KVO
// notification for both pounds and kilos, etc. E.g.:

+ (NSSet *) keyPathsForValuesAffectingPounds
{
   return [NSSet setWithObject:@"kilos"];
}

@end

Now you can set the value as one unit, read it as another, and get KVO notifications for all properties whenever one is set.

You'll want to add constructors (e.g. newWeightWithKilos:), maybe operations (e.g. addWeight: - which can just add the internal values), and need to decide whether a Weight is mutable or immutable.