How do I allow both kg and lbs units for a field
Create two fields, one for number, second for unit. Optionally put them in a field  collection.

Then, add Computed field that will use both values for computation and store the result the way you need it. For best flexibility, you might want to create two of them, ie field_real_kg and field_real_lbs.


  computed fields are populated with values that you define via PHP code. You may draw on anything available to Drupal, including other fields