If I understood correct, your question is how to setup a product weight that will reflect multiple units that you have, right?
The weight attribute in a product does not use any specific unit (see this link and Magento documentation). It is just a number that will be used for shipping calculations. So the real question is: what unit are you using to calculate your shipping? Then you need to convert your weight to be consistent across all products.
Using your example, suppose you have 2 products:
- Dals: 1000kg
- Rice: 5000lbs
and suppose your shipping carrier uses kg to calculate the shipping price.
In this case, since your shipping needs kg as a unit, your products should have the following weight:
- Dals: 1000 (because this is already in kg)
- Rice: 2267.962 (5000 lbs converted to kg)
Magento will not convert this for you (as far as I know), so you need to make the conversion by yourself.