Measurement range versus calibration in electronic instrumentation
In general it would not be acceptable to measure outside of the calibration range, that would mean you'll be extrapolating and you can not guarantee the uncertainty of your measurement anymore.

But it all depends on the accuracy you need to get. 

Based on how your question is phrased I imagine you have a project requirement to have your instruments calibrated by a third party with certificates and all that. If that's the case, you can ask them to give you the uncertainty you would get with their standard and your custom range together with the cost and then decide if you want to pay for  the improved calibration. You might even be surprised to find that the calibration from the manufacturer (so no third party calibration) is better than the lab's standard range.