I've made posts about extending your stamps on several occasions, mostly because I enjoy personalising projects, but also because it's a great way to get more value out of the images you purchase.
The little hippos from the Live Large set are ideal for hand-drawn additions since they have plenty of open space on their bodies. I added a cute floral pattern to Helga using a water-based pen, which won't run or bleed when I colour over the top with alcohol markers.
You can get a similar effect by printing or stamping images onto patterned paper and cutting around them with a pair of small, sharp scissors.
Stamps: