While any competent adult can make almost any agreement (other than one for an illegal purpose) with any other competent adult or entity, with or without a lawyer, whether or not one should is a totally separate matter.
A pre-incorporation agreement sounds as if it would be good to have, and it would be a lot more clear than an oral understanding. Yet if the business venture is going to grow and prosper to be something worthwhile -- or on the other hand if it fails and creditors start to come after the principals -- it's likely that self drawn agreement will, with the benefit of hindsight, have so many holes in it that the resulting litigation likely would could cost multiple times what it would have cost to have had a lawyer prepare -- or at least review -- the agreement in the first place.