We cannot tell from the question whether you are talking about a follow-on grant or adding effort to an existing grant.  We will assume this is a research grant with an educational or non-profit organization.  If it is for something different then the answer could change.  Follow-on grants (awarded without merit based competition) are only done when there is a statute that supersedes the law that requires competition.  We do add some effort to existing grants when it makes sense under the original scope of the grant.  In those cases we get a budget from the university, a description of the work and documentation from the mission partner that the effort fits under the original scope.  We write a short memo describing what we are doing and execute the mod.  We do not typically send to legal unless there is some reason the Grants Officer is concerned about it, e.g. it is a different type of funding than what was used with the original award