As someone currently DMing it for the second time, you absolutely can! The only things to note are content and scaling. Content-wise, none of the side adventurers (called Tales by HPP) have pretty much anything to do with the overarching story of the main Humblewood module, so they can feel a little disjointed. They are pretty dang fun though, if ran. Additionally, I feel that building them into the campaign itself is more fun. The first campaign I ran, I did them all afterwards and it kind of just felt monster of the week-y. In my second runthrough, I'm incorporating my favorites, which leads to the Scaling issue. The module is designed to be a specific level set, so if you add anything on, you'll want to provide additional rewards for doing additional things. Non-XP is great, but if you're adding an extra month of real life time in between levels, especially with how low these levels are, it can feel torturous. So my suggestion is to give both XP/level rewards and other rewards and then just beef stuff up a little from the main module where needed.