No, the advice given by Michael is correct – all insurance must be through RSA Motability, i.e. the insurance included within the lease.
If you aren’t named on the policy (even if you had private insurance), you would likely be in breach of the lease Terms & Conditions. As such, if Motability became aware of what you were doing, they could issue you a Default notice and terminate the agreement = no car, no refund of the advance payment.
Not worth it.