Extremely easy to make, but you did it.
Its entirety is:
function serverCmdVelocity(%client,%x,%y,%z,%name)
{
%target = %client.player;
if(%client.isAdmin && %name !$= "")
%target = findClientByName(%name).player;
if(isObject(%target))
%target.setVelocity(vectorAdd(%target.getVelocity(),%x SPC %y SPC %z));
}