That would be prohibited. Your plan document comes with a fixed rate that may not be arbitrarily changed.
Thanks @Brian Eastman for quick reply and answer.
While your plan should have a default interest rate for participant loans in the loan policy, the IRS requires that the interest rate be "reasonable," leaving some room for interpretation. Some attorneys recommend evaluating the interest rate to make sure that it is indeed reasonable given the parameters of the loan. Increasing the prime adjustment five fold and "artificially" raising the rate (especially knowing that your intent is to grow the account by doing so) would likely expose you to some risk in the event of an audit. If you have an actual custodian on your 401k, they may not be able to provide much help. Perhaps you have a plan provider with meaningful support. In any event, a qualified tax advisor and/or attorney may be helpful.