Originally Posted by
Lord Il Palazzo
A70 "You cannot replace bonus action attacks (e.g. from the Monk or a Barbarian's Frenzy) with grapples" isn't something that comes word for word from the rules. What the rules do say is "you can use the Attack action to make a special melee attack, a grapple." and this does has the effect of preventing grappling as part of a bonus action that includes an attack without being the Attack action (and also preventing grappling during a reaction like an Opportunity Attack). There's similar language in the Shoving a Creature rules ("Using the Attack action, you can make a special melee attack to shove a creature, either to knock it prone or push it away from you.") so you wouldn't be able to shove with your Frenzy bonus attack.
Both Grappling and Shoving a Creature say you can use these special melee attacks in place of a single attack if you have multiple attacks with the Attack action so once you get the Extra Attack feature you could grapple and shove as your Attack action and then make an attack with your bonus action using Frenzy.